Merge branch 'linus' into WIP.x86/boot, to fix up conflicts and to pick up updates

 Conflicts:
	arch/x86/xen/setup.c

Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 5067 insertions, 0 deletions

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
new file mode 100644
index 000000000000..e8f87bf9840c
--- /dev/null
+++ b/kernel/cgroup/cgroup.c
@@ -0,0 +1,5067 @@
+/*
+ *  Generic process-grouping system.
+ *
+ *  Based originally on the cpuset system, extracted by Paul Menage
+ *  Copyright (C) 2006 Google, Inc
+ *
+ *  Notifications support
+ *  Copyright (C) 2009 Nokia Corporation
+ *  Author: Kirill A. Shutemov
+ *
+ *  Copyright notices from the original cpuset code:
+ *  --------------------------------------------------
+ *  Copyright (C) 2003 BULL SA.
+ *  Copyright (C) 2004-2006 Silicon Graphics, Inc.
+ *
+ *  Portions derived from Patrick Mochel's sysfs code.
+ *  sysfs is Copyright (c) 2001-3 Patrick Mochel
+ *
+ *  2003-10-10 Written by Simon Derr.
+ *  2003-10-22 Updates by Stephen Hemminger.
+ *  2004 May-July Rework by Paul Jackson.
+ *  ---------------------------------------------------
+ *
+ *  This file is subject to the terms and conditions of the GNU General Public
+ *  License.  See the file COPYING in the main directory of the Linux
+ *  distribution for more details.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include "cgroup-internal.h"
+
+#include <linux/cred.h>
+#include <linux/errno.h>
+#include <linux/init_task.h>
+#include <linux/kernel.h>
+#include <linux/magic.h>
+#include <linux/mutex.h>
+#include <linux/mount.h>
+#include <linux/pagemap.h>
+#include <linux/proc_fs.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/percpu-rwsem.h>
+#include <linux/string.h>
+#include <linux/hashtable.h>
+#include <linux/idr.h>
+#include <linux/kthread.h>
+#include <linux/atomic.h>
+#include <linux/cpuset.h>
+#include <linux/proc_ns.h>
+#include <linux/nsproxy.h>
+#include <linux/file.h>
+#include <net/sock.h>
+
+#define CREATE_TRACE_POINTS
+#include <trace/events/cgroup.h>
+
+#define CGROUP_FILE_NAME_MAX		(MAX_CGROUP_TYPE_NAMELEN +	\
+					 MAX_CFTYPE_NAME + 2)
+
+/*
+ * cgroup_mutex is the master lock.  Any modification to cgroup or its
+ * hierarchy must be performed while holding it.
+ *
+ * css_set_lock protects task->cgroups pointer, the list of css_set
+ * objects, and the chain of tasks off each css_set.
+ *
+ * These locks are exported if CONFIG_PROVE_RCU so that accessors in
+ * cgroup.h can use them for lockdep annotations.
+ */
+DEFINE_MUTEX(cgroup_mutex);
+DEFINE_SPINLOCK(css_set_lock);
+
+#ifdef CONFIG_PROVE_RCU
+EXPORT_SYMBOL_GPL(cgroup_mutex);
+EXPORT_SYMBOL_GPL(css_set_lock);
+#endif
+
+/*
+ * Protects cgroup_idr and css_idr so that IDs can be released without
+ * grabbing cgroup_mutex.
+ */
+static DEFINE_SPINLOCK(cgroup_idr_lock);
+
+/*
+ * Protects cgroup_file->kn for !self csses.  It synchronizes notifications
+ * against file removal/re-creation across css hiding.
+ */
+static DEFINE_SPINLOCK(cgroup_file_kn_lock);
+
+struct percpu_rw_semaphore cgroup_threadgroup_rwsem;
+
+#define cgroup_assert_mutex_or_rcu_locked()				\
+	RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&			\
+			   !lockdep_is_held(&cgroup_mutex),		\
+			   "cgroup_mutex or RCU read lock required");
+
+/*
+ * cgroup destruction makes heavy use of work items and there can be a lot
+ * of concurrent destructions.  Use a separate workqueue so that cgroup
+ * destruction work items don't end up filling up max_active of system_wq
+ * which may lead to deadlock.
+ */
+static struct workqueue_struct *cgroup_destroy_wq;
+
+/* generate an array of cgroup subsystem pointers */
+#define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys,
+struct cgroup_subsys *cgroup_subsys[] = {
+#include <linux/cgroup_subsys.h>
+};
+#undef SUBSYS
+
+/* array of cgroup subsystem names */
+#define SUBSYS(_x) [_x ## _cgrp_id] = #_x,
+static const char *cgroup_subsys_name[] = {
+#include <linux/cgroup_subsys.h>
+};
+#undef SUBSYS
+
+/* array of static_keys for cgroup_subsys_enabled() and cgroup_subsys_on_dfl() */
+#define SUBSYS(_x)								\
+	DEFINE_STATIC_KEY_TRUE(_x ## _cgrp_subsys_enabled_key);			\
+	DEFINE_STATIC_KEY_TRUE(_x ## _cgrp_subsys_on_dfl_key);			\
+	EXPORT_SYMBOL_GPL(_x ## _cgrp_subsys_enabled_key);			\
+	EXPORT_SYMBOL_GPL(_x ## _cgrp_subsys_on_dfl_key);
+#include <linux/cgroup_subsys.h>
+#undef SUBSYS
+
+#define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys_enabled_key,
+static struct static_key_true *cgroup_subsys_enabled_key[] = {
+#include <linux/cgroup_subsys.h>
+};
+#undef SUBSYS
+
+#define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys_on_dfl_key,
+static struct static_key_true *cgroup_subsys_on_dfl_key[] = {
+#include <linux/cgroup_subsys.h>
+};
+#undef SUBSYS
+
+/*
+ * The default hierarchy, reserved for the subsystems that are otherwise
+ * unattached - it never has more than a single cgroup, and all tasks are
+ * part of that cgroup.
+ */
+struct cgroup_root cgrp_dfl_root;
+EXPORT_SYMBOL_GPL(cgrp_dfl_root);
+
+/*
+ * The default hierarchy always exists but is hidden until mounted for the
+ * first time.  This is for backward compatibility.
+ */
+static bool cgrp_dfl_visible;
+
+/* some controllers are not supported in the default hierarchy */
+static u16 cgrp_dfl_inhibit_ss_mask;
+
+/* some controllers are implicitly enabled on the default hierarchy */
+static u16 cgrp_dfl_implicit_ss_mask;
+
+/* The list of hierarchy roots */
+LIST_HEAD(cgroup_roots);
+static int cgroup_root_count;
+
+/* hierarchy ID allocation and mapping, protected by cgroup_mutex */
+static DEFINE_IDR(cgroup_hierarchy_idr);
+
+/*
+ * Assign a monotonically increasing serial number to csses.  It guarantees
+ * cgroups with bigger numbers are newer than those with smaller numbers.
+ * Also, as csses are always appended to the parent's ->children list, it
+ * guarantees that sibling csses are always sorted in the ascending serial
+ * number order on the list.  Protected by cgroup_mutex.
+ */
+static u64 css_serial_nr_next = 1;
+
+/*
+ * These bitmasks identify subsystems with specific features to avoid
+ * having to do iterative checks repeatedly.
+ */
+static u16 have_fork_callback __read_mostly;
+static u16 have_exit_callback __read_mostly;
+static u16 have_free_callback __read_mostly;
+static u16 have_canfork_callback __read_mostly;
+
+/* cgroup namespace for init task */
+struct cgroup_namespace init_cgroup_ns = {
+	.count		= { .counter = 2, },
+	.user_ns	= &init_user_ns,
+	.ns.ops		= &cgroupns_operations,
+	.ns.inum	= PROC_CGROUP_INIT_INO,
+	.root_cset	= &init_css_set,
+};
+
+static struct file_system_type cgroup2_fs_type;
+static struct cftype cgroup_base_files[];
+
+static int cgroup_apply_control(struct cgroup *cgrp);
+static void cgroup_finalize_control(struct cgroup *cgrp, int ret);
+static void css_task_iter_advance(struct css_task_iter *it);
+static int cgroup_destroy_locked(struct cgroup *cgrp);
+static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
+					      struct cgroup_subsys *ss);
+static void css_release(struct percpu_ref *ref);
+static void kill_css(struct cgroup_subsys_state *css);
+static int cgroup_addrm_files(struct cgroup_subsys_state *css,
+			      struct cgroup *cgrp, struct cftype cfts[],
+			      bool is_add);
+
+/**
+ * cgroup_ssid_enabled - cgroup subsys enabled test by subsys ID
+ * @ssid: subsys ID of interest
+ *
+ * cgroup_subsys_enabled() can only be used with literal subsys names which
+ * is fine for individual subsystems but unsuitable for cgroup core.  This
+ * is slower static_key_enabled() based test indexed by @ssid.
+ */
+bool cgroup_ssid_enabled(int ssid)
+{
+	if (CGROUP_SUBSYS_COUNT == 0)
+		return false;
+
+	return static_key_enabled(cgroup_subsys_enabled_key[ssid]);
+}
+
+/**
+ * cgroup_on_dfl - test whether a cgroup is on the default hierarchy
+ * @cgrp: the cgroup of interest
+ *
+ * The default hierarchy is the v2 interface of cgroup and this function
+ * can be used to test whether a cgroup is on the default hierarchy for
+ * cases where a subsystem should behave differnetly depending on the
+ * interface version.
+ *
+ * The set of behaviors which change on the default hierarchy are still
+ * being determined and the mount option is prefixed with __DEVEL__.
+ *
+ * List of changed behaviors:
+ *
+ * - Mount options "noprefix", "xattr", "clone_children", "release_agent"
+ *   and "name" are disallowed.
+ *
+ * - When mounting an existing superblock, mount options should match.
+ *
+ * - Remount is disallowed.
+ *
+ * - rename(2) is disallowed.
+ *
+ * - "tasks" is removed.  Everything should be at process granularity.  Use
+ *   "cgroup.procs" instead.
+ *
+ * - "cgroup.procs" is not sorted.  pids will be unique unless they got
+ *   recycled inbetween reads.
+ *
+ * - "release_agent" and "notify_on_release" are removed.  Replacement
+ *   notification mechanism will be implemented.
+ *
+ * - "cgroup.clone_children" is removed.
+ *
+ * - "cgroup.subtree_populated" is available.  Its value is 0 if the cgroup
+ *   and its descendants contain no task; otherwise, 1.  The file also
+ *   generates kernfs notification which can be monitored through poll and
+ *   [di]notify when the value of the file changes.
+ *
+ * - cpuset: tasks will be kept in empty cpusets when hotplug happens and
+ *   take masks of ancestors with non-empty cpus/mems, instead of being
+ *   moved to an ancestor.
+ *
+ * - cpuset: a task can be moved into an empty cpuset, and again it takes
+ *   masks of ancestors.
+ *
+ * - memcg: use_hierarchy is on by default and the cgroup file for the flag
+ *   is not created.
+ *
+ * - blkcg: blk-throttle becomes properly hierarchical.
+ *
+ * - debug: disallowed on the default hierarchy.
+ */
+bool cgroup_on_dfl(const struct cgroup *cgrp)
+{
+	return cgrp->root == &cgrp_dfl_root;
+}
+
+/* IDR wrappers which synchronize using cgroup_idr_lock */
+static int cgroup_idr_alloc(struct idr *idr, void *ptr, int start, int end,
+			    gfp_t gfp_mask)
+{
+	int ret;
+
+	idr_preload(gfp_mask);
+	spin_lock_bh(&cgroup_idr_lock);
+	ret = idr_alloc(idr, ptr, start, end, gfp_mask & ~__GFP_DIRECT_RECLAIM);
+	spin_unlock_bh(&cgroup_idr_lock);
+	idr_preload_end();
+	return ret;
+}
+
+static void *cgroup_idr_replace(struct idr *idr, void *ptr, int id)
+{
+	void *ret;
+
+	spin_lock_bh(&cgroup_idr_lock);
+	ret = idr_replace(idr, ptr, id);
+	spin_unlock_bh(&cgroup_idr_lock);
+	return ret;
+}
+
+static void cgroup_idr_remove(struct idr *idr, int id)
+{
+	spin_lock_bh(&cgroup_idr_lock);
+	idr_remove(idr, id);
+	spin_unlock_bh(&cgroup_idr_lock);
+}
+
+static struct cgroup *cgroup_parent(struct cgroup *cgrp)
+{
+	struct cgroup_subsys_state *parent_css = cgrp->self.parent;
+
+	if (parent_css)
+		return container_of(parent_css, struct cgroup, self);
+	return NULL;
+}
+
+/* subsystems visibly enabled on a cgroup */
+static u16 cgroup_control(struct cgroup *cgrp)
+{
+	struct cgroup *parent = cgroup_parent(cgrp);
+	u16 root_ss_mask = cgrp->root->subsys_mask;
+
+	if (parent)
+		return parent->subtree_control;
+
+	if (cgroup_on_dfl(cgrp))
+		root_ss_mask &= ~(cgrp_dfl_inhibit_ss_mask |
+				  cgrp_dfl_implicit_ss_mask);
+	return root_ss_mask;
+}
+
+/* subsystems enabled on a cgroup */
+static u16 cgroup_ss_mask(struct cgroup *cgrp)
+{
+	struct cgroup *parent = cgroup_parent(cgrp);
+
+	if (parent)
+		return parent->subtree_ss_mask;
+
+	return cgrp->root->subsys_mask;
+}
+
+/**
+ * cgroup_css - obtain a cgroup's css for the specified subsystem
+ * @cgrp: the cgroup of interest
+ * @ss: the subsystem of interest (%NULL returns @cgrp->self)
+ *
+ * Return @cgrp's css (cgroup_subsys_state) associated with @ss.  This
+ * function must be called either under cgroup_mutex or rcu_read_lock() and
+ * the caller is responsible for pinning the returned css if it wants to
+ * keep accessing it outside the said locks.  This function may return
+ * %NULL if @cgrp doesn't have @subsys_id enabled.
+ */
+static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
+					      struct cgroup_subsys *ss)
+{
+	if (ss)
+		return rcu_dereference_check(cgrp->subsys[ss->id],
+					lockdep_is_held(&cgroup_mutex));
+	else
+		return &cgrp->self;
+}
+
+/**
+ * cgroup_e_css - obtain a cgroup's effective css for the specified subsystem
+ * @cgrp: the cgroup of interest
+ * @ss: the subsystem of interest (%NULL returns @cgrp->self)
+ *
+ * Similar to cgroup_css() but returns the effective css, which is defined
+ * as the matching css of the nearest ancestor including self which has @ss
+ * enabled.  If @ss is associated with the hierarchy @cgrp is on, this
+ * function is guaranteed to return non-NULL css.
+ */
+static struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
+						struct cgroup_subsys *ss)
+{
+	lockdep_assert_held(&cgroup_mutex);
+
+	if (!ss)
+		return &cgrp->self;
+
+	/*
+	 * This function is used while updating css associations and thus
+	 * can't test the csses directly.  Test ss_mask.
+	 */
+	while (!(cgroup_ss_mask(cgrp) & (1 << ss->id))) {
+		cgrp = cgroup_parent(cgrp);
+		if (!cgrp)
+			return NULL;
+	}
+
+	return cgroup_css(cgrp, ss);
+}
+
+/**
+ * cgroup_get_e_css - get a cgroup's effective css for the specified subsystem
+ * @cgrp: the cgroup of interest
+ * @ss: the subsystem of interest
+ *
+ * Find and get the effective css of @cgrp for @ss.  The effective css is
+ * defined as the matching css of the nearest ancestor including self which
+ * has @ss enabled.  If @ss is not mounted on the hierarchy @cgrp is on,
+ * the root css is returned, so this function always returns a valid css.
+ * The returned css must be put using css_put().
+ */
+struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgrp,
+					     struct cgroup_subsys *ss)
+{
+	struct cgroup_subsys_state *css;
+
+	rcu_read_lock();
+
+	do {
+		css = cgroup_css(cgrp, ss);
+
+		if (css && css_tryget_online(css))
+			goto out_unlock;
+		cgrp = cgroup_parent(cgrp);
+	} while (cgrp);
+
+	css = init_css_set.subsys[ss->id];
+	css_get(css);
+out_unlock:
+	rcu_read_unlock();
+	return css;
+}
+
+static void cgroup_get(struct cgroup *cgrp)
+{
+	WARN_ON_ONCE(cgroup_is_dead(cgrp));
+	css_get(&cgrp->self);
+}
+
+static bool cgroup_tryget(struct cgroup *cgrp)
+{
+	return css_tryget(&cgrp->self);
+}
+
+struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
+{
+	struct cgroup *cgrp = of->kn->parent->priv;
+	struct cftype *cft = of_cft(of);
+
+	/*
+	 * This is open and unprotected implementation of cgroup_css().
+	 * seq_css() is only called from a kernfs file operation which has
+	 * an active reference on the file.  Because all the subsystem
+	 * files are drained before a css is disassociated with a cgroup,
+	 * the matching css from the cgroup's subsys table is guaranteed to
+	 * be and stay valid until the enclosing operation is complete.
+	 */
+	if (cft->ss)
+		return rcu_dereference_raw(cgrp->subsys[cft->ss->id]);
+	else
+		return &cgrp->self;
+}
+EXPORT_SYMBOL_GPL(of_css);
+
+/**
+ * for_each_css - iterate all css's of a cgroup
+ * @css: the iteration cursor
+ * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end
+ * @cgrp: the target cgroup to iterate css's of
+ *
+ * Should be called under cgroup_[tree_]mutex.
+ */
+#define for_each_css(css, ssid, cgrp)					\
+	for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)	\
+		if (!((css) = rcu_dereference_check(			\
+				(cgrp)->subsys[(ssid)],			\
+				lockdep_is_held(&cgroup_mutex)))) { }	\
+		else
+
+/**
+ * for_each_e_css - iterate all effective css's of a cgroup
+ * @css: the iteration cursor
+ * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end
+ * @cgrp: the target cgroup to iterate css's of
+ *
+ * Should be called under cgroup_[tree_]mutex.
+ */
+#define for_each_e_css(css, ssid, cgrp)					\
+	for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)	\
+		if (!((css) = cgroup_e_css(cgrp, cgroup_subsys[(ssid)]))) \
+			;						\
+		else
+
+/**
+ * do_each_subsys_mask - filter for_each_subsys with a bitmask
+ * @ss: the iteration cursor
+ * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
+ * @ss_mask: the bitmask
+ *
+ * The block will only run for cases where the ssid-th bit (1 << ssid) of
+ * @ss_mask is set.
+ */
+#define do_each_subsys_mask(ss, ssid, ss_mask) do {			\
+	unsigned long __ss_mask = (ss_mask);				\
+	if (!CGROUP_SUBSYS_COUNT) { /* to avoid spurious gcc warning */	\
+		(ssid) = 0;						\
+		break;							\
+	}								\
+	for_each_set_bit(ssid, &__ss_mask, CGROUP_SUBSYS_COUNT) {	\
+		(ss) = cgroup_subsys[ssid];				\
+		{
+
+#define while_each_subsys_mask()					\
+		}							\
+	}								\
+} while (false)
+
+/* iterate over child cgrps, lock should be held throughout iteration */
+#define cgroup_for_each_live_child(child, cgrp)				\
+	list_for_each_entry((child), &(cgrp)->self.children, self.sibling) \
+		if (({ lockdep_assert_held(&cgroup_mutex);		\
+		       cgroup_is_dead(child); }))			\
+			;						\
+		else
+
+/* walk live descendants in preorder */
+#define cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp)		\
+	css_for_each_descendant_pre((d_css), cgroup_css((cgrp), NULL))	\
+		if (({ lockdep_assert_held(&cgroup_mutex);		\
+		       (dsct) = (d_css)->cgroup;			\
+		       cgroup_is_dead(dsct); }))			\
+			;						\
+		else
+
+/* walk live descendants in postorder */
+#define cgroup_for_each_live_descendant_post(dsct, d_css, cgrp)		\
+	css_for_each_descendant_post((d_css), cgroup_css((cgrp), NULL))	\
+		if (({ lockdep_assert_held(&cgroup_mutex);		\
+		       (dsct) = (d_css)->cgroup;			\
+		       cgroup_is_dead(dsct); }))			\
+			;						\
+		else
+
+/*
+ * The default css_set - used by init and its children prior to any
+ * hierarchies being mounted. It contains a pointer to the root state
+ * for each subsystem. Also used to anchor the list of css_sets. Not
+ * reference-counted, to improve performance when child cgroups
+ * haven't been created.
+ */
+struct css_set init_css_set = {
+	.refcount		= ATOMIC_INIT(1),
+	.tasks			= LIST_HEAD_INIT(init_css_set.tasks),
+	.mg_tasks		= LIST_HEAD_INIT(init_css_set.mg_tasks),
+	.task_iters		= LIST_HEAD_INIT(init_css_set.task_iters),
+	.cgrp_links		= LIST_HEAD_INIT(init_css_set.cgrp_links),
+	.mg_preload_node	= LIST_HEAD_INIT(init_css_set.mg_preload_node),
+	.mg_node		= LIST_HEAD_INIT(init_css_set.mg_node),
+};
+
+static int css_set_count	= 1;	/* 1 for init_css_set */
+
+/**
+ * css_set_populated - does a css_set contain any tasks?
+ * @cset: target css_set
+ */
+static bool css_set_populated(struct css_set *cset)
+{
+	lockdep_assert_held(&css_set_lock);
+
+	return !list_empty(&cset->tasks) || !list_empty(&cset->mg_tasks);
+}
+
+/**
+ * cgroup_update_populated - updated populated count of a cgroup
+ * @cgrp: the target cgroup
+ * @populated: inc or dec populated count
+ *
+ * One of the css_sets associated with @cgrp is either getting its first
+ * task or losing the last.  Update @cgrp->populated_cnt accordingly.  The
+ * count is propagated towards root so that a given cgroup's populated_cnt
+ * is zero iff the cgroup and all its descendants don't contain any tasks.
+ *
+ * @cgrp's interface file "cgroup.populated" is zero if
+ * @cgrp->populated_cnt is zero and 1 otherwise.  When @cgrp->populated_cnt
+ * changes from or to zero, userland is notified that the content of the
+ * interface file has changed.  This can be used to detect when @cgrp and
+ * its descendants become populated or empty.
+ */
+static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
+{
+	lockdep_assert_held(&css_set_lock);
+
+	do {
+		bool trigger;
+
+		if (populated)
+			trigger = !cgrp->populated_cnt++;
+		else
+			trigger = !--cgrp->populated_cnt;
+
+		if (!trigger)
+			break;
+
+		cgroup1_check_for_release(cgrp);
+		cgroup_file_notify(&cgrp->events_file);
+
+		cgrp = cgroup_parent(cgrp);
+	} while (cgrp);
+}
+
+/**
+ * css_set_update_populated - update populated state of a css_set
+ * @cset: target css_set
+ * @populated: whether @cset is populated or depopulated
+ *
+ * @cset is either getting the first task or losing the last.  Update the
+ * ->populated_cnt of all associated cgroups accordingly.
+ */
+static void css_set_update_populated(struct css_set *cset, bool populated)
+{
+	struct cgrp_cset_link *link;
+
+	lockdep_assert_held(&css_set_lock);
+
+	list_for_each_entry(link, &cset->cgrp_links, cgrp_link)
+		cgroup_update_populated(link->cgrp, populated);
+}
+
+/**
+ * css_set_move_task - move a task from one css_set to another
+ * @task: task being moved
+ * @from_cset: css_set @task currently belongs to (may be NULL)
+ * @to_cset: new css_set @task is being moved to (may be NULL)
+ * @use_mg_tasks: move to @to_cset->mg_tasks instead of ->tasks
+ *
+ * Move @task from @from_cset to @to_cset.  If @task didn't belong to any
+ * css_set, @from_cset can be NULL.  If @task is being disassociated
+ * instead of moved, @to_cset can be NULL.
+ *
+ * This function automatically handles populated_cnt updates and
+ * css_task_iter adjustments but the caller is responsible for managing
+ * @from_cset and @to_cset's reference counts.
+ */
+static void css_set_move_task(struct task_struct *task,
+			      struct css_set *from_cset, struct css_set *to_cset,
+			      bool use_mg_tasks)
+{
+	lockdep_assert_held(&css_set_lock);
+
+	if (to_cset && !css_set_populated(to_cset))
+		css_set_update_populated(to_cset, true);
+
+	if (from_cset) {
+		struct css_task_iter *it, *pos;
+
+		WARN_ON_ONCE(list_empty(&task->cg_list));
+
+		/*
+		 * @task is leaving, advance task iterators which are
+		 * pointing to it so that they can resume at the next
+		 * position.  Advancing an iterator might remove it from
+		 * the list, use safe walk.  See css_task_iter_advance*()
+		 * for details.
+		 */
+		list_for_each_entry_safe(it, pos, &from_cset->task_iters,
+					 iters_node)
+			if (it->task_pos == &task->cg_list)
+				css_task_iter_advance(it);
+
+		list_del_init(&task->cg_list);
+		if (!css_set_populated(from_cset))
+			css_set_update_populated(from_cset, false);
+	} else {
+		WARN_ON_ONCE(!list_empty(&task->cg_list));
+	}
+
+	if (to_cset) {
+		/*
+		 * We are synchronized through cgroup_threadgroup_rwsem
+		 * against PF_EXITING setting such that we can't race
+		 * against cgroup_exit() changing the css_set to
+		 * init_css_set and dropping the old one.
+		 */
+		WARN_ON_ONCE(task->flags & PF_EXITING);
+
+		rcu_assign_pointer(task->cgroups, to_cset);
+		list_add_tail(&task->cg_list, use_mg_tasks ? &to_cset->mg_tasks :
+							     &to_cset->tasks);
+	}
+}
+
+/*
+ * hash table for cgroup groups. This improves the performance to find
+ * an existing css_set. This hash doesn't (currently) take into
+ * account cgroups in empty hierarchies.
+ */
+#define CSS_SET_HASH_BITS	7
+static DEFINE_HASHTABLE(css_set_table, CSS_SET_HASH_BITS);
+
+static unsigned long css_set_hash(struct cgroup_subsys_state *css[])
+{
+	unsigned long key = 0UL;
+	struct cgroup_subsys *ss;
+	int i;
+
+	for_each_subsys(ss, i)
+		key += (unsigned long)css[i];
+	key = (key >> 16) ^ key;
+
+	return key;
+}
+
+void put_css_set_locked(struct css_set *cset)
+{
+	struct cgrp_cset_link *link, *tmp_link;
+	struct cgroup_subsys *ss;
+	int ssid;
+
+	lockdep_assert_held(&css_set_lock);
+
+	if (!atomic_dec_and_test(&cset->refcount))
+		return;
+
+	/* This css_set is dead. unlink it and release cgroup and css refs */
+	for_each_subsys(ss, ssid) {
+		list_del(&cset->e_cset_node[ssid]);
+		css_put(cset->subsys[ssid]);
+	}
+	hash_del(&cset->hlist);
+	css_set_count--;
+
+	list_for_each_entry_safe(link, tmp_link, &cset->cgrp_links, cgrp_link) {
+		list_del(&link->cset_link);
+		list_del(&link->cgrp_link);
+		if (cgroup_parent(link->cgrp))
+			cgroup_put(link->cgrp);
+		kfree(link);
+	}
+
+	kfree_rcu(cset, rcu_head);
+}
+
+/**
+ * compare_css_sets - helper function for find_existing_css_set().
+ * @cset: candidate css_set being tested
+ * @old_cset: existing css_set for a task
+ * @new_cgrp: cgroup that's being entered by the task
+ * @template: desired set of css pointers in css_set (pre-calculated)
+ *
+ * Returns true if "cset" matches "old_cset" except for the hierarchy
+ * which "new_cgrp" belongs to, for which it should match "new_cgrp".
+ */
+static bool compare_css_sets(struct css_set *cset,
+			     struct css_set *old_cset,
+			     struct cgroup *new_cgrp,
+			     struct cgroup_subsys_state *template[])
+{
+	struct list_head *l1, *l2;
+
+	/*
+	 * On the default hierarchy, there can be csets which are
+	 * associated with the same set of cgroups but different csses.
+	 * Let's first ensure that csses match.
+	 */
+	if (memcmp(template, cset->subsys, sizeof(cset->subsys)))
+		return false;
+
+	/*
+	 * Compare cgroup pointers in order to distinguish between
+	 * different cgroups in hierarchies.  As different cgroups may
+	 * share the same effective css, this comparison is always
+	 * necessary.
+	 */
+	l1 = &cset->cgrp_links;
+	l2 = &old_cset->cgrp_links;
+	while (1) {
+		struct cgrp_cset_link *link1, *link2;
+		struct cgroup *cgrp1, *cgrp2;
+
+		l1 = l1->next;
+		l2 = l2->next;
+		/* See if we reached the end - both lists are equal length. */
+		if (l1 == &cset->cgrp_links) {
+			BUG_ON(l2 != &old_cset->cgrp_links);
+			break;
+		} else {
+			BUG_ON(l2 == &old_cset->cgrp_links);
+		}
+		/* Locate the cgroups associated with these links. */
+		link1 = list_entry(l1, struct cgrp_cset_link, cgrp_link);
+		link2 = list_entry(l2, struct cgrp_cset_link, cgrp_link);
+		cgrp1 = link1->cgrp;
+		cgrp2 = link2->cgrp;
+		/* Hierarchies should be linked in the same order. */
+		BUG_ON(cgrp1->root != cgrp2->root);
+
+		/*
+		 * If this hierarchy is the hierarchy of the cgroup
+		 * that's changing, then we need to check that this
+		 * css_set points to the new cgroup; if it's any other
+		 * hierarchy, then this css_set should point to the
+		 * same cgroup as the old css_set.
+		 */
+		if (cgrp1->root == new_cgrp->root) {
+			if (cgrp1 != new_cgrp)
+				return false;
+		} else {
+			if (cgrp1 != cgrp2)
+				return false;
+		}
+	}
+	return true;
+}
+
+/**
+ * find_existing_css_set - init css array and find the matching css_set
+ * @old_cset: the css_set that we're using before the cgroup transition
+ * @cgrp: the cgroup that we're moving into
+ * @template: out param for the new set of csses, should be clear on entry
+ */
+static struct css_set *find_existing_css_set(struct css_set *old_cset,
+					struct cgroup *cgrp,
+					struct cgroup_subsys_state *template[])
+{
+	struct cgroup_root *root = cgrp->root;
+	struct cgroup_subsys *ss;
+	struct css_set *cset;
+	unsigned long key;
+	int i;
+
+	/*
+	 * Build the set of subsystem state objects that we want to see in the
+	 * new css_set. while subsystems can change globally, the entries here
+	 * won't change, so no need for locking.
+	 */
+	for_each_subsys(ss, i) {
+		if (root->subsys_mask & (1UL << i)) {
+			/*
+			 * @ss is in this hierarchy, so we want the
+			 * effective css from @cgrp.
+			 */
+			template[i] = cgroup_e_css(cgrp, ss);
+		} else {
+			/*
+			 * @ss is not in this hierarchy, so we don't want
+			 * to change the css.
+			 */
+			template[i] = old_cset->subsys[i];
+		}
+	}
+
+	key = css_set_hash(template);
+	hash_for_each_possible(css_set_table, cset, hlist, key) {
+		if (!compare_css_sets(cset, old_cset, cgrp, template))
+			continue;
+
+		/* This css_set matches what we need */
+		return cset;
+	}
+
+	/* No existing cgroup group matched */
+	return NULL;
+}
+
+static void free_cgrp_cset_links(struct list_head *links_to_free)
+{
+	struct cgrp_cset_link *link, *tmp_link;
+
+	list_for_each_entry_safe(link, tmp_link, links_to_free, cset_link) {
+		list_del(&link->cset_link);
+		kfree(link);
+	}
+}
+
+/**
+ * allocate_cgrp_cset_links - allocate cgrp_cset_links
+ * @count: the number of links to allocate
+ * @tmp_links: list_head the allocated links are put on
+ *
+ * Allocate @count cgrp_cset_link structures and chain them on @tmp_links
+ * through ->cset_link.  Returns 0 on success or -errno.
+ */
+static int allocate_cgrp_cset_links(int count, struct list_head *tmp_links)
+{
+	struct cgrp_cset_link *link;
+	int i;
+
+	INIT_LIST_HEAD(tmp_links);
+
+	for (i = 0; i < count; i++) {
+		link = kzalloc(sizeof(*link), GFP_KERNEL);
+		if (!link) {
+			free_cgrp_cset_links(tmp_links);
+			return -ENOMEM;
+		}
+		list_add(&link->cset_link, tmp_links);
+	}
+	return 0;
+}
+
+/**
+ * link_css_set - a helper function to link a css_set to a cgroup
+ * @tmp_links: cgrp_cset_link objects allocated by allocate_cgrp_cset_links()
+ * @cset: the css_set to be linked
+ * @cgrp: the destination cgroup
+ */
+static void link_css_set(struct list_head *tmp_links, struct css_set *cset,
+			 struct cgroup *cgrp)
+{
+	struct cgrp_cset_link *link;
+
+	BUG_ON(list_empty(tmp_links));
+
+	if (cgroup_on_dfl(cgrp))
+		cset->dfl_cgrp = cgrp;
+
+	link = list_first_entry(tmp_links, struct cgrp_cset_link, cset_link);
+	link->cset = cset;
+	link->cgrp = cgrp;
+
+	/*
+	 * Always add links to the tail of the lists so that the lists are
+	 * in choronological order.
+	 */
+	list_move_tail(&link->cset_link, &cgrp->cset_links);
+	list_add_tail(&link->cgrp_link, &cset->cgrp_links);
+
+	if (cgroup_parent(cgrp))
+		cgroup_get(cgrp);
+}
+
+/**
+ * find_css_set - return a new css_set with one cgroup updated
+ * @old_cset: the baseline css_set
+ * @cgrp: the cgroup to be updated
+ *
+ * Return a new css_set that's equivalent to @old_cset, but with @cgrp
+ * substituted into the appropriate hierarchy.
+ */
+static struct css_set *find_css_set(struct css_set *old_cset,
+				    struct cgroup *cgrp)
+{
+	struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT] = { };
+	struct css_set *cset;
+	struct list_head tmp_links;
+	struct cgrp_cset_link *link;
+	struct cgroup_subsys *ss;
+	unsigned long key;
+	int ssid;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	/* First see if we already have a cgroup group that matches
+	 * the desired set */
+	spin_lock_irq(&css_set_lock);
+	cset = find_existing_css_set(old_cset, cgrp, template);
+	if (cset)
+		get_css_set(cset);
+	spin_unlock_irq(&css_set_lock);
+
+	if (cset)
+		return cset;
+
+	cset = kzalloc(sizeof(*cset), GFP_KERNEL);
+	if (!cset)
+		return NULL;
+
+	/* Allocate all the cgrp_cset_link objects that we'll need */
+	if (allocate_cgrp_cset_links(cgroup_root_count, &tmp_links) < 0) {
+		kfree(cset);
+		return NULL;
+	}
+
+	atomic_set(&cset->refcount, 1);
+	INIT_LIST_HEAD(&cset->tasks);
+	INIT_LIST_HEAD(&cset->mg_tasks);
+	INIT_LIST_HEAD(&cset->task_iters);
+	INIT_HLIST_NODE(&cset->hlist);
+	INIT_LIST_HEAD(&cset->cgrp_links);
+	INIT_LIST_HEAD(&cset->mg_preload_node);
+	INIT_LIST_HEAD(&cset->mg_node);
+
+	/* Copy the set of subsystem state objects generated in
+	 * find_existing_css_set() */
+	memcpy(cset->subsys, template, sizeof(cset->subsys));
+
+	spin_lock_irq(&css_set_lock);
+	/* Add reference counts and links from the new css_set. */
+	list_for_each_entry(link, &old_cset->cgrp_links, cgrp_link) {
+		struct cgroup *c = link->cgrp;
+
+		if (c->root == cgrp->root)
+			c = cgrp;
+		link_css_set(&tmp_links, cset, c);
+	}
+
+	BUG_ON(!list_empty(&tmp_links));
+
+	css_set_count++;
+
+	/* Add @cset to the hash table */
+	key = css_set_hash(cset->subsys);
+	hash_add(css_set_table, &cset->hlist, key);
+
+	for_each_subsys(ss, ssid) {
+		struct cgroup_subsys_state *css = cset->subsys[ssid];
+
+		list_add_tail(&cset->e_cset_node[ssid],
+			      &css->cgroup->e_csets[ssid]);
+		css_get(css);
+	}
+
+	spin_unlock_irq(&css_set_lock);
+
+	return cset;
+}
+
+struct cgroup_root *cgroup_root_from_kf(struct kernfs_root *kf_root)
+{
+	struct cgroup *root_cgrp = kf_root->kn->priv;
+
+	return root_cgrp->root;
+}
+
+static int cgroup_init_root_id(struct cgroup_root *root)
+{
+	int id;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, 0, 0, GFP_KERNEL);
+	if (id < 0)
+		return id;
+
+	root->hierarchy_id = id;
+	return 0;
+}
+
+static void cgroup_exit_root_id(struct cgroup_root *root)
+{
+	lockdep_assert_held(&cgroup_mutex);
+
+	idr_remove(&cgroup_hierarchy_idr, root->hierarchy_id);
+}
+
+void cgroup_free_root(struct cgroup_root *root)
+{
+	if (root) {
+		idr_destroy(&root->cgroup_idr);
+		kfree(root);
+	}
+}
+
+static void cgroup_destroy_root(struct cgroup_root *root)
+{
+	struct cgroup *cgrp = &root->cgrp;
+	struct cgrp_cset_link *link, *tmp_link;
+
+	trace_cgroup_destroy_root(root);
+
+	cgroup_lock_and_drain_offline(&cgrp_dfl_root.cgrp);
+
+	BUG_ON(atomic_read(&root->nr_cgrps));
+	BUG_ON(!list_empty(&cgrp->self.children));
+
+	/* Rebind all subsystems back to the default hierarchy */
+	WARN_ON(rebind_subsystems(&cgrp_dfl_root, root->subsys_mask));
+
+	/*
+	 * Release all the links from cset_links to this hierarchy's
+	 * root cgroup
+	 */
+	spin_lock_irq(&css_set_lock);
+
+	list_for_each_entry_safe(link, tmp_link, &cgrp->cset_links, cset_link) {
+		list_del(&link->cset_link);
+		list_del(&link->cgrp_link);
+		kfree(link);
+	}
+
+	spin_unlock_irq(&css_set_lock);
+
+	if (!list_empty(&root->root_list)) {
+		list_del(&root->root_list);
+		cgroup_root_count--;
+	}
+
+	cgroup_exit_root_id(root);
+
+	mutex_unlock(&cgroup_mutex);
+
+	kernfs_destroy_root(root->kf_root);
+	cgroup_free_root(root);
+}
+
+/*
+ * look up cgroup associated with current task's cgroup namespace on the
+ * specified hierarchy
+ */
+static struct cgroup *
+current_cgns_cgroup_from_root(struct cgroup_root *root)
+{
+	struct cgroup *res = NULL;
+	struct css_set *cset;
+
+	lockdep_assert_held(&css_set_lock);
+
+	rcu_read_lock();
+
+	cset = current->nsproxy->cgroup_ns->root_cset;
+	if (cset == &init_css_set) {
+		res = &root->cgrp;
+	} else {
+		struct cgrp_cset_link *link;
+
+		list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
+			struct cgroup *c = link->cgrp;
+
+			if (c->root == root) {
+				res = c;
+				break;
+			}
+		}
+	}
+	rcu_read_unlock();
+
+	BUG_ON(!res);
+	return res;
+}
+
+/* look up cgroup associated with given css_set on the specified hierarchy */
+static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
+					    struct cgroup_root *root)
+{
+	struct cgroup *res = NULL;
+
+	lockdep_assert_held(&cgroup_mutex);
+	lockdep_assert_held(&css_set_lock);
+
+	if (cset == &init_css_set) {
+		res = &root->cgrp;
+	} else {
+		struct cgrp_cset_link *link;
+
+		list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
+			struct cgroup *c = link->cgrp;
+
+			if (c->root == root) {
+				res = c;
+				break;
+			}
+		}
+	}
+
+	BUG_ON(!res);
+	return res;
+}
+
+/*
+ * Return the cgroup for "task" from the given hierarchy. Must be
+ * called with cgroup_mutex and css_set_lock held.
+ */
+struct cgroup *task_cgroup_from_root(struct task_struct *task,
+				     struct cgroup_root *root)
+{
+	/*
+	 * No need to lock the task - since we hold cgroup_mutex the
+	 * task can't change groups, so the only thing that can happen
+	 * is that it exits and its css is set back to init_css_set.
+	 */
+	return cset_cgroup_from_root(task_css_set(task), root);
+}
+
+/*
+ * A task must hold cgroup_mutex to modify cgroups.
+ *
+ * Any task can increment and decrement the count field without lock.
+ * So in general, code holding cgroup_mutex can't rely on the count
+ * field not changing.  However, if the count goes to zero, then only
+ * cgroup_attach_task() can increment it again.  Because a count of zero
+ * means that no tasks are currently attached, therefore there is no
+ * way a task attached to that cgroup can fork (the other way to
+ * increment the count).  So code holding cgroup_mutex can safely
+ * assume that if the count is zero, it will stay zero. Similarly, if
+ * a task holds cgroup_mutex on a cgroup with zero count, it
+ * knows that the cgroup won't be removed, as cgroup_rmdir()
+ * needs that mutex.
+ *
+ * A cgroup can only be deleted if both its 'count' of using tasks
+ * is zero, and its list of 'children' cgroups is empty.  Since all
+ * tasks in the system use _some_ cgroup, and since there is always at
+ * least one task in the system (init, pid == 1), therefore, root cgroup
+ * always has either children cgroups and/or using tasks.  So we don't
+ * need a special hack to ensure that root cgroup cannot be deleted.
+ *
+ * P.S.  One more locking exception.  RCU is used to guard the
+ * update of a tasks cgroup pointer by cgroup_attach_task()
+ */
+
+static struct kernfs_syscall_ops cgroup_kf_syscall_ops;
+
+static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
+			      char *buf)
+{
+	struct cgroup_subsys *ss = cft->ss;
+
+	if (cft->ss && !(cft->flags & CFTYPE_NO_PREFIX) &&
+	    !(cgrp->root->flags & CGRP_ROOT_NOPREFIX))
+		snprintf(buf, CGROUP_FILE_NAME_MAX, "%s.%s",
+			 cgroup_on_dfl(cgrp) ? ss->name : ss->legacy_name,
+			 cft->name);
+	else
+		strncpy(buf, cft->name, CGROUP_FILE_NAME_MAX);
+	return buf;
+}
+
+/**
+ * cgroup_file_mode - deduce file mode of a control file
+ * @cft: the control file in question
+ *
+ * S_IRUGO for read, S_IWUSR for write.
+ */
+static umode_t cgroup_file_mode(const struct cftype *cft)
+{
+	umode_t mode = 0;
+
+	if (cft->read_u64 || cft->read_s64 || cft->seq_show)
+		mode |= S_IRUGO;
+
+	if (cft->write_u64 || cft->write_s64 || cft->write) {
+		if (cft->flags & CFTYPE_WORLD_WRITABLE)
+			mode |= S_IWUGO;
+		else
+			mode |= S_IWUSR;
+	}
+
+	return mode;
+}
+
+/**
+ * cgroup_calc_subtree_ss_mask - calculate subtree_ss_mask
+ * @subtree_control: the new subtree_control mask to consider
+ * @this_ss_mask: available subsystems
+ *
+ * On the default hierarchy, a subsystem may request other subsystems to be
+ * enabled together through its ->depends_on mask.  In such cases, more
+ * subsystems than specified in "cgroup.subtree_control" may be enabled.
+ *
+ * This function calculates which subsystems need to be enabled if
+ * @subtree_control is to be applied while restricted to @this_ss_mask.
+ */
+static u16 cgroup_calc_subtree_ss_mask(u16 subtree_control, u16 this_ss_mask)
+{
+	u16 cur_ss_mask = subtree_control;
+	struct cgroup_subsys *ss;
+	int ssid;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	cur_ss_mask |= cgrp_dfl_implicit_ss_mask;
+
+	while (true) {
+		u16 new_ss_mask = cur_ss_mask;
+
+		do_each_subsys_mask(ss, ssid, cur_ss_mask) {
+			new_ss_mask |= ss->depends_on;
+		} while_each_subsys_mask();
+
+		/*
+		 * Mask out subsystems which aren't available.  This can
+		 * happen only if some depended-upon subsystems were bound
+		 * to non-default hierarchies.
+		 */
+		new_ss_mask &= this_ss_mask;
+
+		if (new_ss_mask == cur_ss_mask)
+			break;
+		cur_ss_mask = new_ss_mask;
+	}
+
+	return cur_ss_mask;
+}
+
+/**
+ * cgroup_kn_unlock - unlocking helper for cgroup kernfs methods
+ * @kn: the kernfs_node being serviced
+ *
+ * This helper undoes cgroup_kn_lock_live() and should be invoked before
+ * the method finishes if locking succeeded.  Note that once this function
+ * returns the cgroup returned by cgroup_kn_lock_live() may become
+ * inaccessible any time.  If the caller intends to continue to access the
+ * cgroup, it should pin it before invoking this function.
+ */
+void cgroup_kn_unlock(struct kernfs_node *kn)
+{
+	struct cgroup *cgrp;
+
+	if (kernfs_type(kn) == KERNFS_DIR)
+		cgrp = kn->priv;
+	else
+		cgrp = kn->parent->priv;
+
+	mutex_unlock(&cgroup_mutex);
+
+	kernfs_unbreak_active_protection(kn);
+	cgroup_put(cgrp);
+}
+
+/**
+ * cgroup_kn_lock_live - locking helper for cgroup kernfs methods
+ * @kn: the kernfs_node being serviced
+ * @drain_offline: perform offline draining on the cgroup
+ *
+ * This helper is to be used by a cgroup kernfs method currently servicing
+ * @kn.  It breaks the active protection, performs cgroup locking and
+ * verifies that the associated cgroup is alive.  Returns the cgroup if
+ * alive; otherwise, %NULL.  A successful return should be undone by a
+ * matching cgroup_kn_unlock() invocation.  If @drain_offline is %true, the
+ * cgroup is drained of offlining csses before return.
+ *
+ * Any cgroup kernfs method implementation which requires locking the
+ * associated cgroup should use this helper.  It avoids nesting cgroup
+ * locking under kernfs active protection and allows all kernfs operations
+ * including self-removal.
+ */
+struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn, bool drain_offline)
+{
+	struct cgroup *cgrp;
+
+	if (kernfs_type(kn) == KERNFS_DIR)
+		cgrp = kn->priv;
+	else
+		cgrp = kn->parent->priv;
+
+	/*
+	 * We're gonna grab cgroup_mutex which nests outside kernfs
+	 * active_ref.  cgroup liveliness check alone provides enough
+	 * protection against removal.  Ensure @cgrp stays accessible and
+	 * break the active_ref protection.
+	 */
+	if (!cgroup_tryget(cgrp))
+		return NULL;
+	kernfs_break_active_protection(kn);
+
+	if (drain_offline)
+		cgroup_lock_and_drain_offline(cgrp);
+	else
+		mutex_lock(&cgroup_mutex);
+
+	if (!cgroup_is_dead(cgrp))
+		return cgrp;
+
+	cgroup_kn_unlock(kn);
+	return NULL;
+}
+
+static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
+{
+	char name[CGROUP_FILE_NAME_MAX];
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	if (cft->file_offset) {
+		struct cgroup_subsys_state *css = cgroup_css(cgrp, cft->ss);
+		struct cgroup_file *cfile = (void *)css + cft->file_offset;
+
+		spin_lock_irq(&cgroup_file_kn_lock);
+		cfile->kn = NULL;
+		spin_unlock_irq(&cgroup_file_kn_lock);
+	}
+
+	kernfs_remove_by_name(cgrp->kn, cgroup_file_name(cgrp, cft, name));
+}
+
+/**
+ * css_clear_dir - remove subsys files in a cgroup directory
+ * @css: taget css
+ */
+static void css_clear_dir(struct cgroup_subsys_state *css)
+{
+	struct cgroup *cgrp = css->cgroup;
+	struct cftype *cfts;
+
+	if (!(css->flags & CSS_VISIBLE))
+		return;
+
+	css->flags &= ~CSS_VISIBLE;
+
+	list_for_each_entry(cfts, &css->ss->cfts, node)
+		cgroup_addrm_files(css, cgrp, cfts, false);
+}
+
+/**
+ * css_populate_dir - create subsys files in a cgroup directory
+ * @css: target css
+ *
+ * On failure, no file is added.
+ */
+static int css_populate_dir(struct cgroup_subsys_state *css)
+{
+	struct cgroup *cgrp = css->cgroup;
+	struct cftype *cfts, *failed_cfts;
+	int ret;
+
+	if ((css->flags & CSS_VISIBLE) || !cgrp->kn)
+		return 0;
+
+	if (!css->ss) {
+		if (cgroup_on_dfl(cgrp))
+			cfts = cgroup_base_files;
+		else
+			cfts = cgroup1_base_files;
+
+		return cgroup_addrm_files(&cgrp->self, cgrp, cfts, true);
+	}
+
+	list_for_each_entry(cfts, &css->ss->cfts, node) {
+		ret = cgroup_addrm_files(css, cgrp, cfts, true);
+		if (ret < 0) {
+			failed_cfts = cfts;
+			goto err;
+		}
+	}
+
+	css->flags |= CSS_VISIBLE;
+
+	return 0;
+err:
+	list_for_each_entry(cfts, &css->ss->cfts, node) {
+		if (cfts == failed_cfts)
+			break;
+		cgroup_addrm_files(css, cgrp, cfts, false);
+	}
+	return ret;
+}
+
+int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
+{
+	struct cgroup *dcgrp = &dst_root->cgrp;
+	struct cgroup_subsys *ss;
+	int ssid, i, ret;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	do_each_subsys_mask(ss, ssid, ss_mask) {
+		/*
+		 * If @ss has non-root csses attached to it, can't move.
+		 * If @ss is an implicit controller, it is exempt from this
+		 * rule and can be stolen.
+		 */
+		if (css_next_child(NULL, cgroup_css(&ss->root->cgrp, ss)) &&
+		    !ss->implicit_on_dfl)
+			return -EBUSY;
+
+		/* can't move between two non-dummy roots either */
+		if (ss->root != &cgrp_dfl_root && dst_root != &cgrp_dfl_root)
+			return -EBUSY;
+	} while_each_subsys_mask();
+
+	do_each_subsys_mask(ss, ssid, ss_mask) {
+		struct cgroup_root *src_root = ss->root;
+		struct cgroup *scgrp = &src_root->cgrp;
+		struct cgroup_subsys_state *css = cgroup_css(scgrp, ss);
+		struct css_set *cset;
+
+		WARN_ON(!css || cgroup_css(dcgrp, ss));
+
+		/* disable from the source */
+		src_root->subsys_mask &= ~(1 << ssid);
+		WARN_ON(cgroup_apply_control(scgrp));
+		cgroup_finalize_control(scgrp, 0);
+
+		/* rebind */
+		RCU_INIT_POINTER(scgrp->subsys[ssid], NULL);
+		rcu_assign_pointer(dcgrp->subsys[ssid], css);
+		ss->root = dst_root;
+		css->cgroup = dcgrp;
+
+		spin_lock_irq(&css_set_lock);
+		hash_for_each(css_set_table, i, cset, hlist)
+			list_move_tail(&cset->e_cset_node[ss->id],
+				       &dcgrp->e_csets[ss->id]);
+		spin_unlock_irq(&css_set_lock);
+
+		/* default hierarchy doesn't enable controllers by default */
+		dst_root->subsys_mask |= 1 << ssid;
+		if (dst_root == &cgrp_dfl_root) {
+			static_branch_enable(cgroup_subsys_on_dfl_key[ssid]);
+		} else {
+			dcgrp->subtree_control |= 1 << ssid;
+			static_branch_disable(cgroup_subsys_on_dfl_key[ssid]);
+		}
+
+		ret = cgroup_apply_control(dcgrp);
+		if (ret)
+			pr_warn("partial failure to rebind %s controller (err=%d)\n",
+				ss->name, ret);
+
+		if (ss->bind)
+			ss->bind(css);
+	} while_each_subsys_mask();
+
+	kernfs_activate(dcgrp->kn);
+	return 0;
+}
+
+int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
+		     struct kernfs_root *kf_root)
+{
+	int len = 0;
+	char *buf = NULL;
+	struct cgroup_root *kf_cgroot = cgroup_root_from_kf(kf_root);
+	struct cgroup *ns_cgroup;
+
+	buf = kmalloc(PATH_MAX, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	spin_lock_irq(&css_set_lock);
+	ns_cgroup = current_cgns_cgroup_from_root(kf_cgroot);
+	len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX);
+	spin_unlock_irq(&css_set_lock);
+
+	if (len >= PATH_MAX)
+		len = -ERANGE;
+	else if (len > 0) {
+		seq_escape(sf, buf, " \t\n\\");
+		len = 0;
+	}
+	kfree(buf);
+	return len;
+}
+
+static int cgroup_remount(struct kernfs_root *kf_root, int *flags, char *data)
+{
+	pr_err("remount is not allowed\n");
+	return -EINVAL;
+}
+
+/*
+ * To reduce the fork() overhead for systems that are not actually using
+ * their cgroups capability, we don't maintain the lists running through
+ * each css_set to its tasks until we see the list actually used - in other
+ * words after the first mount.
+ */
+static bool use_task_css_set_links __read_mostly;
+
+static void cgroup_enable_task_cg_lists(void)
+{
+	struct task_struct *p, *g;
+
+	spin_lock_irq(&css_set_lock);
+
+	if (use_task_css_set_links)
+		goto out_unlock;
+
+	use_task_css_set_links = true;
+
+	/*
+	 * We need tasklist_lock because RCU is not safe against
+	 * while_each_thread(). Besides, a forking task that has passed
+	 * cgroup_post_fork() without seeing use_task_css_set_links = 1
+	 * is not guaranteed to have its child immediately visible in the
+	 * tasklist if we walk through it with RCU.
+	 */
+	read_lock(&tasklist_lock);
+	do_each_thread(g, p) {
+		WARN_ON_ONCE(!list_empty(&p->cg_list) ||
+			     task_css_set(p) != &init_css_set);
+
+		/*
+		 * We should check if the process is exiting, otherwise
+		 * it will race with cgroup_exit() in that the list
+		 * entry won't be deleted though the process has exited.
+		 * Do it while holding siglock so that we don't end up
+		 * racing against cgroup_exit().
+		 *
+		 * Interrupts were already disabled while acquiring
+		 * the css_set_lock, so we do not need to disable it
+		 * again when acquiring the sighand->siglock here.
+		 */
+		spin_lock(&p->sighand->siglock);
+		if (!(p->flags & PF_EXITING)) {
+			struct css_set *cset = task_css_set(p);
+
+			if (!css_set_populated(cset))
+				css_set_update_populated(cset, true);
+			list_add_tail(&p->cg_list, &cset->tasks);
+			get_css_set(cset);
+		}
+		spin_unlock(&p->sighand->siglock);
+	} while_each_thread(g, p);
+	read_unlock(&tasklist_lock);
+out_unlock:
+	spin_unlock_irq(&css_set_lock);
+}
+
+static void init_cgroup_housekeeping(struct cgroup *cgrp)
+{
+	struct cgroup_subsys *ss;
+	int ssid;
+
+	INIT_LIST_HEAD(&cgrp->self.sibling);
+	INIT_LIST_HEAD(&cgrp->self.children);
+	INIT_LIST_HEAD(&cgrp->cset_links);
+	INIT_LIST_HEAD(&cgrp->pidlists);
+	mutex_init(&cgrp->pidlist_mutex);
+	cgrp->self.cgroup = cgrp;
+	cgrp->self.flags |= CSS_ONLINE;
+
+	for_each_subsys(ss, ssid)
+		INIT_LIST_HEAD(&cgrp->e_csets[ssid]);
+
+	init_waitqueue_head(&cgrp->offline_waitq);
+	INIT_WORK(&cgrp->release_agent_work, cgroup1_release_agent);
+}
+
+void init_cgroup_root(struct cgroup_root *root, struct cgroup_sb_opts *opts)
+{
+	struct cgroup *cgrp = &root->cgrp;
+
+	INIT_LIST_HEAD(&root->root_list);
+	atomic_set(&root->nr_cgrps, 1);
+	cgrp->root = root;
+	init_cgroup_housekeeping(cgrp);
+	idr_init(&root->cgroup_idr);
+
+	root->flags = opts->flags;
+	if (opts->release_agent)
+		strcpy(root->release_agent_path, opts->release_agent);
+	if (opts->name)
+		strcpy(root->name, opts->name);
+	if (opts->cpuset_clone_children)
+		set_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->cgrp.flags);
+}
+
+int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask)
+{
+	LIST_HEAD(tmp_links);
+	struct cgroup *root_cgrp = &root->cgrp;
+	struct kernfs_syscall_ops *kf_sops;
+	struct css_set *cset;
+	int i, ret;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	ret = cgroup_idr_alloc(&root->cgroup_idr, root_cgrp, 1, 2, GFP_KERNEL);
+	if (ret < 0)
+		goto out;
+	root_cgrp->id = ret;
+	root_cgrp->ancestor_ids[0] = ret;
+
+	ret = percpu_ref_init(&root_cgrp->self.refcnt, css_release, 0,
+			      GFP_KERNEL);
+	if (ret)
+		goto out;
+
+	/*
+	 * We're accessing css_set_count without locking css_set_lock here,
+	 * but that's OK - it can only be increased by someone holding
+	 * cgroup_lock, and that's us.  Later rebinding may disable
+	 * controllers on the default hierarchy and thus create new csets,
+	 * which can't be more than the existing ones.  Allocate 2x.
+	 */
+	ret = allocate_cgrp_cset_links(2 * css_set_count, &tmp_links);
+	if (ret)
+		goto cancel_ref;
+
+	ret = cgroup_init_root_id(root);
+	if (ret)
+		goto cancel_ref;
+
+	kf_sops = root == &cgrp_dfl_root ?
+		&cgroup_kf_syscall_ops : &cgroup1_kf_syscall_ops;
+
+	root->kf_root = kernfs_create_root(kf_sops,
+					   KERNFS_ROOT_CREATE_DEACTIVATED,
+					   root_cgrp);
+	if (IS_ERR(root->kf_root)) {
+		ret = PTR_ERR(root->kf_root);
+		goto exit_root_id;
+	}
+	root_cgrp->kn = root->kf_root->kn;
+
+	ret = css_populate_dir(&root_cgrp->self);
+	if (ret)
+		goto destroy_root;
+
+	ret = rebind_subsystems(root, ss_mask);
+	if (ret)
+		goto destroy_root;
+
+	trace_cgroup_setup_root(root);
+
+	/*
+	 * There must be no failure case after here, since rebinding takes
+	 * care of subsystems' refcounts, which are explicitly dropped in
+	 * the failure exit path.
+	 */
+	list_add(&root->root_list, &cgroup_roots);
+	cgroup_root_count++;
+
+	/*
+	 * Link the root cgroup in this hierarchy into all the css_set
+	 * objects.
+	 */
+	spin_lock_irq(&css_set_lock);
+	hash_for_each(css_set_table, i, cset, hlist) {
+		link_css_set(&tmp_links, cset, root_cgrp);
+		if (css_set_populated(cset))
+			cgroup_update_populated(root_cgrp, true);
+	}
+	spin_unlock_irq(&css_set_lock);
+
+	BUG_ON(!list_empty(&root_cgrp->self.children));
+	BUG_ON(atomic_read(&root->nr_cgrps) != 1);
+
+	kernfs_activate(root_cgrp->kn);
+	ret = 0;
+	goto out;
+
+destroy_root:
+	kernfs_destroy_root(root->kf_root);
+	root->kf_root = NULL;
+exit_root_id:
+	cgroup_exit_root_id(root);
+cancel_ref:
+	percpu_ref_exit(&root_cgrp->self.refcnt);
+out:
+	free_cgrp_cset_links(&tmp_links);
+	return ret;
+}
+
+struct dentry *cgroup_do_mount(struct file_system_type *fs_type, int flags,
+			       struct cgroup_root *root, unsigned long magic,
+			       struct cgroup_namespace *ns)
+{
+	struct dentry *dentry;
+	bool new_sb;
+
+	dentry = kernfs_mount(fs_type, flags, root->kf_root, magic, &new_sb);
+
+	/*
+	 * In non-init cgroup namespace, instead of root cgroup's dentry,
+	 * we return the dentry corresponding to the cgroupns->root_cgrp.
+	 */
+	if (!IS_ERR(dentry) && ns != &init_cgroup_ns) {
+		struct dentry *nsdentry;
+		struct cgroup *cgrp;
+
+		mutex_lock(&cgroup_mutex);
+		spin_lock_irq(&css_set_lock);
+
+		cgrp = cset_cgroup_from_root(ns->root_cset, root);
+
+		spin_unlock_irq(&css_set_lock);
+		mutex_unlock(&cgroup_mutex);
+
+		nsdentry = kernfs_node_dentry(cgrp->kn, dentry->d_sb);
+		dput(dentry);
+		dentry = nsdentry;
+	}
+
+	if (IS_ERR(dentry) || !new_sb)
+		cgroup_put(&root->cgrp);
+
+	return dentry;
+}
+
+static struct dentry *cgroup_mount(struct file_system_type *fs_type,
+			 int flags, const char *unused_dev_name,
+			 void *data)
+{
+	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
+	struct dentry *dentry;
+
+	get_cgroup_ns(ns);
+
+	/* Check if the caller has permission to mount. */
+	if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN)) {
+		put_cgroup_ns(ns);
+		return ERR_PTR(-EPERM);
+	}
+
+	/*
+	 * The first time anyone tries to mount a cgroup, enable the list
+	 * linking each css_set to its tasks and fix up all existing tasks.
+	 */
+	if (!use_task_css_set_links)
+		cgroup_enable_task_cg_lists();
+
+	if (fs_type == &cgroup2_fs_type) {
+		if (data) {
+			pr_err("cgroup2: unknown option \"%s\"\n", (char *)data);
+			put_cgroup_ns(ns);
+			return ERR_PTR(-EINVAL);
+		}
+		cgrp_dfl_visible = true;
+		cgroup_get(&cgrp_dfl_root.cgrp);
+
+		dentry = cgroup_do_mount(&cgroup2_fs_type, flags, &cgrp_dfl_root,
+					 CGROUP2_SUPER_MAGIC, ns);
+	} else {
+		dentry = cgroup1_mount(&cgroup_fs_type, flags, data,
+				       CGROUP_SUPER_MAGIC, ns);
+	}
+
+	put_cgroup_ns(ns);
+	return dentry;
+}
+
+static void cgroup_kill_sb(struct super_block *sb)
+{
+	struct kernfs_root *kf_root = kernfs_root_from_sb(sb);
+	struct cgroup_root *root = cgroup_root_from_kf(kf_root);
+
+	/*
+	 * If @root doesn't have any mounts or children, start killing it.
+	 * This prevents new mounts by disabling percpu_ref_tryget_live().
+	 * cgroup_mount() may wait for @root's release.
+	 *
+	 * And don't kill the default root.
+	 */
+	if (!list_empty(&root->cgrp.self.children) ||
+	    root == &cgrp_dfl_root)
+		cgroup_put(&root->cgrp);
+	else
+		percpu_ref_kill(&root->cgrp.self.refcnt);
+
+	kernfs_kill_sb(sb);
+}
+
+struct file_system_type cgroup_fs_type = {
+	.name = "cgroup",
+	.mount = cgroup_mount,
+	.kill_sb = cgroup_kill_sb,
+	.fs_flags = FS_USERNS_MOUNT,
+};
+
+static struct file_system_type cgroup2_fs_type = {
+	.name = "cgroup2",
+	.mount = cgroup_mount,
+	.kill_sb = cgroup_kill_sb,
+	.fs_flags = FS_USERNS_MOUNT,
+};
+
+int cgroup_path_ns_locked(struct cgroup *cgrp, char *buf, size_t buflen,
+			  struct cgroup_namespace *ns)
+{
+	struct cgroup *root = cset_cgroup_from_root(ns->root_cset, cgrp->root);
+
+	return kernfs_path_from_node(cgrp->kn, root->kn, buf, buflen);
+}
+
+int cgroup_path_ns(struct cgroup *cgrp, char *buf, size_t buflen,
+		   struct cgroup_namespace *ns)
+{
+	int ret;
+
+	mutex_lock(&cgroup_mutex);
+	spin_lock_irq(&css_set_lock);
+
+	ret = cgroup_path_ns_locked(cgrp, buf, buflen, ns);
+
+	spin_unlock_irq(&css_set_lock);
+	mutex_unlock(&cgroup_mutex);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(cgroup_path_ns);
+
+/**
+ * task_cgroup_path - cgroup path of a task in the first cgroup hierarchy
+ * @task: target task
+ * @buf: the buffer to write the path into
+ * @buflen: the length of the buffer
+ *
+ * Determine @task's cgroup on the first (the one with the lowest non-zero
+ * hierarchy_id) cgroup hierarchy and copy its path into @buf.  This
+ * function grabs cgroup_mutex and shouldn't be used inside locks used by
+ * cgroup controller callbacks.
+ *
+ * Return value is the same as kernfs_path().
+ */
+int task_cgroup_path(struct task_struct *task, char *buf, size_t buflen)
+{
+	struct cgroup_root *root;
+	struct cgroup *cgrp;
+	int hierarchy_id = 1;
+	int ret;
+
+	mutex_lock(&cgroup_mutex);
+	spin_lock_irq(&css_set_lock);
+
+	root = idr_get_next(&cgroup_hierarchy_idr, &hierarchy_id);
+
+	if (root) {
+		cgrp = task_cgroup_from_root(task, root);
+		ret = cgroup_path_ns_locked(cgrp, buf, buflen, &init_cgroup_ns);
+	} else {
+		/* if no hierarchy exists, everyone is in "/" */
+		ret = strlcpy(buf, "/", buflen);
+	}
+
+	spin_unlock_irq(&css_set_lock);
+	mutex_unlock(&cgroup_mutex);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(task_cgroup_path);
+
+/**
+ * cgroup_migrate_add_task - add a migration target task to a migration context
+ * @task: target task
+ * @mgctx: target migration context
+ *
+ * Add @task, which is a migration target, to @mgctx->tset.  This function
+ * becomes noop if @task doesn't need to be migrated.  @task's css_set
+ * should have been added as a migration source and @task->cg_list will be
+ * moved from the css_set's tasks list to mg_tasks one.
+ */
+static void cgroup_migrate_add_task(struct task_struct *task,
+				    struct cgroup_mgctx *mgctx)
+{
+	struct css_set *cset;
+
+	lockdep_assert_held(&css_set_lock);
+
+	/* @task either already exited or can't exit until the end */
+	if (task->flags & PF_EXITING)
+		return;
+
+	/* leave @task alone if post_fork() hasn't linked it yet */
+	if (list_empty(&task->cg_list))
+		return;
+
+	cset = task_css_set(task);
+	if (!cset->mg_src_cgrp)
+		return;
+
+	list_move_tail(&task->cg_list, &cset->mg_tasks);
+	if (list_empty(&cset->mg_node))
+		list_add_tail(&cset->mg_node,
+			      &mgctx->tset.src_csets);
+	if (list_empty(&cset->mg_dst_cset->mg_node))
+		list_add_tail(&cset->mg_dst_cset->mg_node,
+			      &mgctx->tset.dst_csets);
+}
+
+/**
+ * cgroup_taskset_first - reset taskset and return the first task
+ * @tset: taskset of interest
+ * @dst_cssp: output variable for the destination css
+ *
+ * @tset iteration is initialized and the first task is returned.
+ */
+struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset,
+					 struct cgroup_subsys_state **dst_cssp)
+{
+	tset->cur_cset = list_first_entry(tset->csets, struct css_set, mg_node);
+	tset->cur_task = NULL;
+
+	return cgroup_taskset_next(tset, dst_cssp);
+}
+
+/**
+ * cgroup_taskset_next - iterate to the next task in taskset
+ * @tset: taskset of interest
+ * @dst_cssp: output variable for the destination css
+ *
+ * Return the next task in @tset.  Iteration must have been initialized
+ * with cgroup_taskset_first().
+ */
+struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset,
+					struct cgroup_subsys_state **dst_cssp)
+{
+	struct css_set *cset = tset->cur_cset;
+	struct task_struct *task = tset->cur_task;
+
+	while (&cset->mg_node != tset->csets) {
+		if (!task)
+			task = list_first_entry(&cset->mg_tasks,
+						struct task_struct, cg_list);
+		else
+			task = list_next_entry(task, cg_list);
+
+		if (&task->cg_list != &cset->mg_tasks) {
+			tset->cur_cset = cset;
+			tset->cur_task = task;
+
+			/*
+			 * This function may be called both before and
+			 * after cgroup_taskset_migrate().  The two cases
+			 * can be distinguished by looking at whether @cset
+			 * has its ->mg_dst_cset set.
+			 */
+			if (cset->mg_dst_cset)
+				*dst_cssp = cset->mg_dst_cset->subsys[tset->ssid];
+			else
+				*dst_cssp = cset->subsys[tset->ssid];
+
+			return task;
+		}
+
+		cset = list_next_entry(cset, mg_node);
+		task = NULL;
+	}
+
+	return NULL;
+}
+
+/**
+ * cgroup_taskset_migrate - migrate a taskset
+ * @mgctx: migration context
+ *
+ * Migrate tasks in @mgctx as setup by migration preparation functions.
+ * This function fails iff one of the ->can_attach callbacks fails and
+ * guarantees that either all or none of the tasks in @mgctx are migrated.
+ * @mgctx is consumed regardless of success.
+ */
+static int cgroup_migrate_execute(struct cgroup_mgctx *mgctx)
+{
+	struct cgroup_taskset *tset = &mgctx->tset;
+	struct cgroup_subsys *ss;
+	struct task_struct *task, *tmp_task;
+	struct css_set *cset, *tmp_cset;
+	int ssid, failed_ssid, ret;
+
+	/* methods shouldn't be called if no task is actually migrating */
+	if (list_empty(&tset->src_csets))
+		return 0;
+
+	/* check that we can legitimately attach to the cgroup */
+	do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
+		if (ss->can_attach) {
+			tset->ssid = ssid;
+			ret = ss->can_attach(tset);
+			if (ret) {
+				failed_ssid = ssid;
+				goto out_cancel_attach;
+			}
+		}
+	} while_each_subsys_mask();
+
+	/*
+	 * Now that we're guaranteed success, proceed to move all tasks to
+	 * the new cgroup.  There are no failure cases after here, so this
+	 * is the commit point.
+	 */
+	spin_lock_irq(&css_set_lock);
+	list_for_each_entry(cset, &tset->src_csets, mg_node) {
+		list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list) {
+			struct css_set *from_cset = task_css_set(task);
+			struct css_set *to_cset = cset->mg_dst_cset;
+
+			get_css_set(to_cset);
+			css_set_move_task(task, from_cset, to_cset, true);
+			put_css_set_locked(from_cset);
+		}
+	}
+	spin_unlock_irq(&css_set_lock);
+
+	/*
+	 * Migration is committed, all target tasks are now on dst_csets.
+	 * Nothing is sensitive to fork() after this point.  Notify
+	 * controllers that migration is complete.
+	 */
+	tset->csets = &tset->dst_csets;
+
+	do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
+		if (ss->attach) {
+			tset->ssid = ssid;
+			ss->attach(tset);
+		}
+	} while_each_subsys_mask();
+
+	ret = 0;
+	goto out_release_tset;
+
+out_cancel_attach:
+	do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
+		if (ssid == failed_ssid)
+			break;
+		if (ss->cancel_attach) {
+			tset->ssid = ssid;
+			ss->cancel_attach(tset);
+		}
+	} while_each_subsys_mask();
+out_release_tset:
+	spin_lock_irq(&css_set_lock);
+	list_splice_init(&tset->dst_csets, &tset->src_csets);
+	list_for_each_entry_safe(cset, tmp_cset, &tset->src_csets, mg_node) {
+		list_splice_tail_init(&cset->mg_tasks, &cset->tasks);
+		list_del_init(&cset->mg_node);
+	}
+	spin_unlock_irq(&css_set_lock);
+	return ret;
+}
+
+/**
+ * cgroup_may_migrate_to - verify whether a cgroup can be migration destination
+ * @dst_cgrp: destination cgroup to test
+ *
+ * On the default hierarchy, except for the root, subtree_control must be
+ * zero for migration destination cgroups with tasks so that child cgroups
+ * don't compete against tasks.
+ */
+bool cgroup_may_migrate_to(struct cgroup *dst_cgrp)
+{
+	return !cgroup_on_dfl(dst_cgrp) || !cgroup_parent(dst_cgrp) ||
+		!dst_cgrp->subtree_control;
+}
+
+/**
+ * cgroup_migrate_finish - cleanup after attach
+ * @mgctx: migration context
+ *
+ * Undo cgroup_migrate_add_src() and cgroup_migrate_prepare_dst().  See
+ * those functions for details.
+ */
+void cgroup_migrate_finish(struct cgroup_mgctx *mgctx)
+{
+	LIST_HEAD(preloaded);
+	struct css_set *cset, *tmp_cset;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	spin_lock_irq(&css_set_lock);
+
+	list_splice_tail_init(&mgctx->preloaded_src_csets, &preloaded);
+	list_splice_tail_init(&mgctx->preloaded_dst_csets, &preloaded);
+
+	list_for_each_entry_safe(cset, tmp_cset, &preloaded, mg_preload_node) {
+		cset->mg_src_cgrp = NULL;
+		cset->mg_dst_cgrp = NULL;
+		cset->mg_dst_cset = NULL;
+		list_del_init(&cset->mg_preload_node);
+		put_css_set_locked(cset);
+	}
+
+	spin_unlock_irq(&css_set_lock);
+}
+
+/**
+ * cgroup_migrate_add_src - add a migration source css_set
+ * @src_cset: the source css_set to add
+ * @dst_cgrp: the destination cgroup
+ * @mgctx: migration context
+ *
+ * Tasks belonging to @src_cset are about to be migrated to @dst_cgrp.  Pin
+ * @src_cset and add it to @mgctx->src_csets, which should later be cleaned
+ * up by cgroup_migrate_finish().
+ *
+ * This function may be called without holding cgroup_threadgroup_rwsem
+ * even if the target is a process.  Threads may be created and destroyed
+ * but as long as cgroup_mutex is not dropped, no new css_set can be put
+ * into play and the preloaded css_sets are guaranteed to cover all
+ * migrations.
+ */
+void cgroup_migrate_add_src(struct css_set *src_cset,
+			    struct cgroup *dst_cgrp,
+			    struct cgroup_mgctx *mgctx)
+{
+	struct cgroup *src_cgrp;
+
+	lockdep_assert_held(&cgroup_mutex);
+	lockdep_assert_held(&css_set_lock);
+
+	/*
+	 * If ->dead, @src_set is associated with one or more dead cgroups
+	 * and doesn't contain any migratable tasks.  Ignore it early so
+	 * that the rest of migration path doesn't get confused by it.
+	 */
+	if (src_cset->dead)
+		return;
+
+	src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);
+
+	if (!list_empty(&src_cset->mg_preload_node))
+		return;
+
+	WARN_ON(src_cset->mg_src_cgrp);
+	WARN_ON(src_cset->mg_dst_cgrp);
+	WARN_ON(!list_empty(&src_cset->mg_tasks));
+	WARN_ON(!list_empty(&src_cset->mg_node));
+
+	src_cset->mg_src_cgrp = src_cgrp;
+	src_cset->mg_dst_cgrp = dst_cgrp;
+	get_css_set(src_cset);
+	list_add_tail(&src_cset->mg_preload_node, &mgctx->preloaded_src_csets);
+}
+
+/**
+ * cgroup_migrate_prepare_dst - prepare destination css_sets for migration
+ * @mgctx: migration context
+ *
+ * Tasks are about to be moved and all the source css_sets have been
+ * preloaded to @mgctx->preloaded_src_csets.  This function looks up and
+ * pins all destination css_sets, links each to its source, and append them
+ * to @mgctx->preloaded_dst_csets.
+ *
+ * This function must be called after cgroup_migrate_add_src() has been
+ * called on each migration source css_set.  After migration is performed
+ * using cgroup_migrate(), cgroup_migrate_finish() must be called on
+ * @mgctx.
+ */
+int cgroup_migrate_prepare_dst(struct cgroup_mgctx *mgctx)
+{
+	struct css_set *src_cset, *tmp_cset;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	/* look up the dst cset for each src cset and link it to src */
+	list_for_each_entry_safe(src_cset, tmp_cset, &mgctx->preloaded_src_csets,
+				 mg_preload_node) {
+		struct css_set *dst_cset;
+		struct cgroup_subsys *ss;
+		int ssid;
+
+		dst_cset = find_css_set(src_cset, src_cset->mg_dst_cgrp);
+		if (!dst_cset)
+			goto err;
+
+		WARN_ON_ONCE(src_cset->mg_dst_cset || dst_cset->mg_dst_cset);
+
+		/*
+		 * If src cset equals dst, it's noop.  Drop the src.
+		 * cgroup_migrate() will skip the cset too.  Note that we
+		 * can't handle src == dst as some nodes are used by both.
+		 */
+		if (src_cset == dst_cset) {
+			src_cset->mg_src_cgrp = NULL;
+			src_cset->mg_dst_cgrp = NULL;
+			list_del_init(&src_cset->mg_preload_node);
+			put_css_set(src_cset);
+			put_css_set(dst_cset);
+			continue;
+		}
+
+		src_cset->mg_dst_cset = dst_cset;
+
+		if (list_empty(&dst_cset->mg_preload_node))
+			list_add_tail(&dst_cset->mg_preload_node,
+				      &mgctx->preloaded_dst_csets);
+		else
+			put_css_set(dst_cset);
+
+		for_each_subsys(ss, ssid)
+			if (src_cset->subsys[ssid] != dst_cset->subsys[ssid])
+				mgctx->ss_mask |= 1 << ssid;
+	}
+
+	return 0;
+err:
+	cgroup_migrate_finish(mgctx);
+	return -ENOMEM;
+}
+
+/**
+ * cgroup_migrate - migrate a process or task to a cgroup
+ * @leader: the leader of the process or the task to migrate
+ * @threadgroup: whether @leader points to the whole process or a single task
+ * @mgctx: migration context
+ *
+ * Migrate a process or task denoted by @leader.  If migrating a process,
+ * the caller must be holding cgroup_threadgroup_rwsem.  The caller is also
+ * responsible for invoking cgroup_migrate_add_src() and
+ * cgroup_migrate_prepare_dst() on the targets before invoking this
+ * function and following up with cgroup_migrate_finish().
+ *
+ * As long as a controller's ->can_attach() doesn't fail, this function is
+ * guaranteed to succeed.  This means that, excluding ->can_attach()
+ * failure, when migrating multiple targets, the success or failure can be
+ * decided for all targets by invoking group_migrate_prepare_dst() before
+ * actually starting migrating.
+ */
+int cgroup_migrate(struct task_struct *leader, bool threadgroup,
+		   struct cgroup_mgctx *mgctx)
+{
+	struct task_struct *task;
+
+	/*
+	 * Prevent freeing of tasks while we take a snapshot. Tasks that are
+	 * already PF_EXITING could be freed from underneath us unless we
+	 * take an rcu_read_lock.
+	 */
+	spin_lock_irq(&css_set_lock);
+	rcu_read_lock();
+	task = leader;
+	do {
+		cgroup_migrate_add_task(task, mgctx);
+		if (!threadgroup)
+			break;
+	} while_each_thread(leader, task);
+	rcu_read_unlock();
+	spin_unlock_irq(&css_set_lock);
+
+	return cgroup_migrate_execute(mgctx);
+}
+
+/**
+ * cgroup_attach_task - attach a task or a whole threadgroup to a cgroup
+ * @dst_cgrp: the cgroup to attach to
+ * @leader: the task or the leader of the threadgroup to be attached
+ * @threadgroup: attach the whole threadgroup?
+ *
+ * Call holding cgroup_mutex and cgroup_threadgroup_rwsem.
+ */
+int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
+		       bool threadgroup)
+{
+	DEFINE_CGROUP_MGCTX(mgctx);
+	struct task_struct *task;
+	int ret;
+
+	if (!cgroup_may_migrate_to(dst_cgrp))
+		return -EBUSY;
+
+	/* look up all src csets */
+	spin_lock_irq(&css_set_lock);
+	rcu_read_lock();
+	task = leader;
+	do {
+		cgroup_migrate_add_src(task_css_set(task), dst_cgrp, &mgctx);
+		if (!threadgroup)
+			break;
+	} while_each_thread(leader, task);
+	rcu_read_unlock();
+	spin_unlock_irq(&css_set_lock);
+
+	/* prepare dst csets and commit */
+	ret = cgroup_migrate_prepare_dst(&mgctx);
+	if (!ret)
+		ret = cgroup_migrate(leader, threadgroup, &mgctx);
+
+	cgroup_migrate_finish(&mgctx);
+
+	if (!ret)
+		trace_cgroup_attach_task(dst_cgrp, leader, threadgroup);
+
+	return ret;
+}
+
+static int cgroup_procs_write_permission(struct task_struct *task,
+					 struct cgroup *dst_cgrp,
+					 struct kernfs_open_file *of)
+{
+	int ret = 0;
+
+	if (cgroup_on_dfl(dst_cgrp)) {
+		struct super_block *sb = of->file->f_path.dentry->d_sb;
+		struct cgroup *cgrp;
+		struct inode *inode;
+
+		spin_lock_irq(&css_set_lock);
+		cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
+		spin_unlock_irq(&css_set_lock);
+
+		while (!cgroup_is_descendant(dst_cgrp, cgrp))
+			cgrp = cgroup_parent(cgrp);
+
+		ret = -ENOMEM;
+		inode = kernfs_get_inode(sb, cgrp->procs_file.kn);
+		if (inode) {
+			ret = inode_permission(inode, MAY_WRITE);
+			iput(inode);
+		}
+	} else {
+		const struct cred *cred = current_cred();
+		const struct cred *tcred = get_task_cred(task);
+
+		/*
+		 * even if we're attaching all tasks in the thread group,
+		 * we only need to check permissions on one of them.
+		 */
+		if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) &&
+		    !uid_eq(cred->euid, tcred->uid) &&
+		    !uid_eq(cred->euid, tcred->suid))
+			ret = -EACCES;
+		put_cred(tcred);
+	}
+
+	return ret;
+}
+
+/*
+ * Find the task_struct of the task to attach by vpid and pass it along to the
+ * function to attach either it or all tasks in its threadgroup. Will lock
+ * cgroup_mutex and threadgroup.
+ */
+ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
+			     size_t nbytes, loff_t off, bool threadgroup)
+{
+	struct task_struct *tsk;
+	struct cgroup_subsys *ss;
+	struct cgroup *cgrp;
+	pid_t pid;
+	int ssid, ret;
+
+	if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
+		return -EINVAL;
+
+	cgrp = cgroup_kn_lock_live(of->kn, false);
+	if (!cgrp)
+		return -ENODEV;
+
+	percpu_down_write(&cgroup_threadgroup_rwsem);
+	rcu_read_lock();
+	if (pid) {
+		tsk = find_task_by_vpid(pid);
+		if (!tsk) {
+			ret = -ESRCH;
+			goto out_unlock_rcu;
+		}
+	} else {
+		tsk = current;
+	}
+
+	if (threadgroup)
+		tsk = tsk->group_leader;
+
+	/*
+	 * Workqueue threads may acquire PF_NO_SETAFFINITY and become
+	 * trapped in a cpuset, or RT worker may be born in a cgroup
+	 * with no rt_runtime allocated.  Just say no.
+	 */
+	if (tsk == kthreadd_task || (tsk->flags & PF_NO_SETAFFINITY)) {
+		ret = -EINVAL;
+		goto out_unlock_rcu;
+	}
+
+	get_task_struct(tsk);
+	rcu_read_unlock();
+
+	ret = cgroup_procs_write_permission(tsk, cgrp, of);
+	if (!ret)
+		ret = cgroup_attach_task(cgrp, tsk, threadgroup);
+
+	put_task_struct(tsk);
+	goto out_unlock_threadgroup;
+
+out_unlock_rcu:
+	rcu_read_unlock();
+out_unlock_threadgroup:
+	percpu_up_write(&cgroup_threadgroup_rwsem);
+	for_each_subsys(ss, ssid)
+		if (ss->post_attach)
+			ss->post_attach();
+	cgroup_kn_unlock(of->kn);
+	return ret ?: nbytes;
+}
+
+ssize_t cgroup_procs_write(struct kernfs_open_file *of, char *buf, size_t nbytes,
+			   loff_t off)
+{
+	return __cgroup_procs_write(of, buf, nbytes, off, true);
+}
+
+static void cgroup_print_ss_mask(struct seq_file *seq, u16 ss_mask)
+{
+	struct cgroup_subsys *ss;
+	bool printed = false;
+	int ssid;
+
+	do_each_subsys_mask(ss, ssid, ss_mask) {
+		if (printed)
+			seq_putc(seq, ' ');
+		seq_printf(seq, "%s", ss->name);
+		printed = true;
+	} while_each_subsys_mask();
+	if (printed)
+		seq_putc(seq, '\n');
+}
+
+/* show controllers which are enabled from the parent */
+static int cgroup_controllers_show(struct seq_file *seq, void *v)
+{
+	struct cgroup *cgrp = seq_css(seq)->cgroup;
+
+	cgroup_print_ss_mask(seq, cgroup_control(cgrp));
+	return 0;
+}
+
+/* show controllers which are enabled for a given cgroup's children */
+static int cgroup_subtree_control_show(struct seq_file *seq, void *v)
+{
+	struct cgroup *cgrp = seq_css(seq)->cgroup;
+
+	cgroup_print_ss_mask(seq, cgrp->subtree_control);
+	return 0;
+}
+
+/**
+ * cgroup_update_dfl_csses - update css assoc of a subtree in default hierarchy
+ * @cgrp: root of the subtree to update csses for
+ *
+ * @cgrp's control masks have changed and its subtree's css associations
+ * need to be updated accordingly.  This function looks up all css_sets
+ * which are attached to the subtree, creates the matching updated css_sets
+ * and migrates the tasks to the new ones.
+ */
+static int cgroup_update_dfl_csses(struct cgroup *cgrp)
+{
+	DEFINE_CGROUP_MGCTX(mgctx);
+	struct cgroup_subsys_state *d_css;
+	struct cgroup *dsct;
+	struct css_set *src_cset;
+	int ret;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	percpu_down_write(&cgroup_threadgroup_rwsem);
+
+	/* look up all csses currently attached to @cgrp's subtree */
+	spin_lock_irq(&css_set_lock);
+	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
+		struct cgrp_cset_link *link;
+
+		list_for_each_entry(link, &dsct->cset_links, cset_link)
+			cgroup_migrate_add_src(link->cset, dsct, &mgctx);
+	}
+	spin_unlock_irq(&css_set_lock);
+
+	/* NULL dst indicates self on default hierarchy */
+	ret = cgroup_migrate_prepare_dst(&mgctx);
+	if (ret)
+		goto out_finish;
+
+	spin_lock_irq(&css_set_lock);
+	list_for_each_entry(src_cset, &mgctx.preloaded_src_csets, mg_preload_node) {
+		struct task_struct *task, *ntask;
+
+		/* all tasks in src_csets need to be migrated */
+		list_for_each_entry_safe(task, ntask, &src_cset->tasks, cg_list)
+			cgroup_migrate_add_task(task, &mgctx);
+	}
+	spin_unlock_irq(&css_set_lock);
+
+	ret = cgroup_migrate_execute(&mgctx);
+out_finish:
+	cgroup_migrate_finish(&mgctx);
+	percpu_up_write(&cgroup_threadgroup_rwsem);
+	return ret;
+}
+
+/**
+ * cgroup_lock_and_drain_offline - lock cgroup_mutex and drain offlined csses
+ * @cgrp: root of the target subtree
+ *
+ * Because css offlining is asynchronous, userland may try to re-enable a
+ * controller while the previous css is still around.  This function grabs
+ * cgroup_mutex and drains the previous css instances of @cgrp's subtree.
+ */
+void cgroup_lock_and_drain_offline(struct cgroup *cgrp)
+	__acquires(&cgroup_mutex)
+{
+	struct cgroup *dsct;
+	struct cgroup_subsys_state *d_css;
+	struct cgroup_subsys *ss;
+	int ssid;
+
+restart:
+	mutex_lock(&cgroup_mutex);
+
+	cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
+		for_each_subsys(ss, ssid) {
+			struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
+			DEFINE_WAIT(wait);
+
+			if (!css || !percpu_ref_is_dying(&css->refcnt))
+				continue;
+
+			cgroup_get(dsct);
+			prepare_to_wait(&dsct->offline_waitq, &wait,
+					TASK_UNINTERRUPTIBLE);
+
+			mutex_unlock(&cgroup_mutex);
+			schedule();
+			finish_wait(&dsct->offline_waitq, &wait);
+
+			cgroup_put(dsct);
+			goto restart;
+		}
+	}
+}
+
+/**
+ * cgroup_save_control - save control masks of a subtree
+ * @cgrp: root of the target subtree
+ *
+ * Save ->subtree_control and ->subtree_ss_mask to the respective old_
+ * prefixed fields for @cgrp's subtree including @cgrp itself.
+ */
+static void cgroup_save_control(struct cgroup *cgrp)
+{
+	struct cgroup *dsct;
+	struct cgroup_subsys_state *d_css;
+
+	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
+		dsct->old_subtree_control = dsct->subtree_control;
+		dsct->old_subtree_ss_mask = dsct->subtree_ss_mask;
+	}
+}
+
+/**
+ * cgroup_propagate_control - refresh control masks of a subtree
+ * @cgrp: root of the target subtree
+ *
+ * For @cgrp and its subtree, ensure ->subtree_ss_mask matches
+ * ->subtree_control and propagate controller availability through the
+ * subtree so that descendants don't have unavailable controllers enabled.
+ */
+static void cgroup_propagate_control(struct cgroup *cgrp)
+{
+	struct cgroup *dsct;
+	struct cgroup_subsys_state *d_css;
+
+	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
+		dsct->subtree_control &= cgroup_control(dsct);
+		dsct->subtree_ss_mask =
+			cgroup_calc_subtree_ss_mask(dsct->subtree_control,
+						    cgroup_ss_mask(dsct));
+	}
+}
+
+/**
+ * cgroup_restore_control - restore control masks of a subtree
+ * @cgrp: root of the target subtree
+ *
+ * Restore ->subtree_control and ->subtree_ss_mask from the respective old_
+ * prefixed fields for @cgrp's subtree including @cgrp itself.
+ */
+static void cgroup_restore_control(struct cgroup *cgrp)
+{
+	struct cgroup *dsct;
+	struct cgroup_subsys_state *d_css;
+
+	cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
+		dsct->subtree_control = dsct->old_subtree_control;
+		dsct->subtree_ss_mask = dsct->old_subtree_ss_mask;
+	}
+}
+
+static bool css_visible(struct cgroup_subsys_state *css)
+{
+	struct cgroup_subsys *ss = css->ss;
+	struct cgroup *cgrp = css->cgroup;
+
+	if (cgroup_control(cgrp) & (1 << ss->id))
+		return true;
+	if (!(cgroup_ss_mask(cgrp) & (1 << ss->id)))
+		return false;
+	return cgroup_on_dfl(cgrp) && ss->implicit_on_dfl;
+}
+
+/**
+ * cgroup_apply_control_enable - enable or show csses according to control
+ * @cgrp: root of the target subtree
+ *
+ * Walk @cgrp's subtree and create new csses or make the existing ones
+ * visible.  A css is created invisible if it's being implicitly enabled
+ * through dependency.  An invisible css is made visible when the userland
+ * explicitly enables it.
+ *
+ * Returns 0 on success, -errno on failure.  On failure, csses which have
+ * been processed already aren't cleaned up.  The caller is responsible for
+ * cleaning up with cgroup_apply_control_disble().
+ */
+static int cgroup_apply_control_enable(struct cgroup *cgrp)
+{
+	struct cgroup *dsct;
+	struct cgroup_subsys_state *d_css;
+	struct cgroup_subsys *ss;
+	int ssid, ret;
+
+	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
+		for_each_subsys(ss, ssid) {
+			struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
+
+			WARN_ON_ONCE(css && percpu_ref_is_dying(&css->refcnt));
+
+			if (!(cgroup_ss_mask(dsct) & (1 << ss->id)))
+				continue;
+
+			if (!css) {
+				css = css_create(dsct, ss);
+				if (IS_ERR(css))
+					return PTR_ERR(css);
+			}
+
+			if (css_visible(css)) {
+				ret = css_populate_dir(css);
+				if (ret)
+					return ret;
+			}
+		}
+	}
+
+	return 0;
+}
+
+/**
+ * cgroup_apply_control_disable - kill or hide csses according to control
+ * @cgrp: root of the target subtree
+ *
+ * Walk @cgrp's subtree and kill and hide csses so that they match
+ * cgroup_ss_mask() and cgroup_visible_mask().
+ *
+ * A css is hidden when the userland requests it to be disabled while other
+ * subsystems are still depending on it.  The css must not actively control
+ * resources and be in the vanilla state if it's made visible again later.
+ * Controllers which may be depended upon should provide ->css_reset() for
+ * this purpose.
+ */
+static void cgroup_apply_control_disable(struct cgroup *cgrp)
+{
+	struct cgroup *dsct;
+	struct cgroup_subsys_state *d_css;
+	struct cgroup_subsys *ss;
+	int ssid;
+
+	cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
+		for_each_subsys(ss, ssid) {
+			struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
+
+			WARN_ON_ONCE(css && percpu_ref_is_dying(&css->refcnt));
+
+			if (!css)
+				continue;
+
+			if (css->parent &&
+			    !(cgroup_ss_mask(dsct) & (1 << ss->id))) {
+				kill_css(css);
+			} else if (!css_visible(css)) {
+				css_clear_dir(css);
+				if (ss->css_reset)
+					ss->css_reset(css);
+			}
+		}
+	}
+}
+
+/**
+ * cgroup_apply_control - apply control mask updates to the subtree
+ * @cgrp: root of the target subtree
+ *
+ * subsystems can be enabled and disabled in a subtree using the following
+ * steps.
+ *
+ * 1. Call cgroup_save_control() to stash the current state.
+ * 2. Update ->subtree_control masks in the subtree as desired.
+ * 3. Call cgroup_apply_control() to apply the changes.
+ * 4. Optionally perform other related operations.
+ * 5. Call cgroup_finalize_control() to finish up.
+ *
+ * This function implements step 3 and propagates the mask changes
+ * throughout @cgrp's subtree, updates csses accordingly and perform
+ * process migrations.
+ */
+static int cgroup_apply_control(struct cgroup *cgrp)
+{
+	int ret;
+
+	cgroup_propagate_control(cgrp);
+
+	ret = cgroup_apply_control_enable(cgrp);
+	if (ret)
+		return ret;
+
+	/*
+	 * At this point, cgroup_e_css() results reflect the new csses
+	 * making the following cgroup_update_dfl_csses() properly update
+	 * css associations of all tasks in the subtree.
+	 */
+	ret = cgroup_update_dfl_csses(cgrp);
+	if (ret)
+		return ret;
+
+	return 0;
+}
+
+/**
+ * cgroup_finalize_control - finalize control mask update
+ * @cgrp: root of the target subtree
+ * @ret: the result of the update
+ *
+ * Finalize control mask update.  See cgroup_apply_control() for more info.
+ */
+static void cgroup_finalize_control(struct cgroup *cgrp, int ret)
+{
+	if (ret) {
+		cgroup_restore_control(cgrp);
+		cgroup_propagate_control(cgrp);
+	}
+
+	cgroup_apply_control_disable(cgrp);
+}
+
+/* change the enabled child controllers for a cgroup in the default hierarchy */
+static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
+					    char *buf, size_t nbytes,
+					    loff_t off)
+{
+	u16 enable = 0, disable = 0;
+	struct cgroup *cgrp, *child;
+	struct cgroup_subsys *ss;
+	char *tok;
+	int ssid, ret;
+
+	/*
+	 * Parse input - space separated list of subsystem names prefixed
+	 * with either + or -.
+	 */
+	buf = strstrip(buf);
+	while ((tok = strsep(&buf, " "))) {
+		if (tok[0] == '\0')
+			continue;
+		do_each_subsys_mask(ss, ssid, ~cgrp_dfl_inhibit_ss_mask) {
+			if (!cgroup_ssid_enabled(ssid) ||
+			    strcmp(tok + 1, ss->name))
+				continue;
+
+			if (*tok == '+') {
+				enable |= 1 << ssid;
+				disable &= ~(1 << ssid);
+			} else if (*tok == '-') {
+				disable |= 1 << ssid;
+				enable &= ~(1 << ssid);
+			} else {
+				return -EINVAL;
+			}
+			break;
+		} while_each_subsys_mask();
+		if (ssid == CGROUP_SUBSYS_COUNT)
+			return -EINVAL;
+	}
+
+	cgrp = cgroup_kn_lock_live(of->kn, true);
+	if (!cgrp)
+		return -ENODEV;
+
+	for_each_subsys(ss, ssid) {
+		if (enable & (1 << ssid)) {
+			if (cgrp->subtree_control & (1 << ssid)) {
+				enable &= ~(1 << ssid);
+				continue;
+			}
+
+			if (!(cgroup_control(cgrp) & (1 << ssid))) {
+				ret = -ENOENT;
+				goto out_unlock;
+			}
+		} else if (disable & (1 << ssid)) {
+			if (!(cgrp->subtree_control & (1 << ssid))) {
+				disable &= ~(1 << ssid);
+				continue;
+			}
+
+			/* a child has it enabled? */
+			cgroup_for_each_live_child(child, cgrp) {
+				if (child->subtree_control & (1 << ssid)) {
+					ret = -EBUSY;
+					goto out_unlock;
+				}
+			}
+		}
+	}
+
+	if (!enable && !disable) {
+		ret = 0;
+		goto out_unlock;
+	}
+
+	/*
+	 * Except for the root, subtree_control must be zero for a cgroup
+	 * with tasks so that child cgroups don't compete against tasks.
+	 */
+	if (enable && cgroup_parent(cgrp)) {
+		struct cgrp_cset_link *link;
+
+		/*
+		 * Because namespaces pin csets too, @cgrp->cset_links
+		 * might not be empty even when @cgrp is empty.  Walk and
+		 * verify each cset.
+		 */
+		spin_lock_irq(&css_set_lock);
+
+		ret = 0;
+		list_for_each_entry(link, &cgrp->cset_links, cset_link) {
+			if (css_set_populated(link->cset)) {
+				ret = -EBUSY;
+				break;
+			}
+		}
+
+		spin_unlock_irq(&css_set_lock);
+
+		if (ret)
+			goto out_unlock;
+	}
+
+	/* save and update control masks and prepare csses */
+	cgroup_save_control(cgrp);
+
+	cgrp->subtree_control |= enable;
+	cgrp->subtree_control &= ~disable;
+
+	ret = cgroup_apply_control(cgrp);
+
+	cgroup_finalize_control(cgrp, ret);
+
+	kernfs_activate(cgrp->kn);
+	ret = 0;
+out_unlock:
+	cgroup_kn_unlock(of->kn);
+	return ret ?: nbytes;
+}
+
+static int cgroup_events_show(struct seq_file *seq, void *v)
+{
+	seq_printf(seq, "populated %d\n",
+		   cgroup_is_populated(seq_css(seq)->cgroup));
+	return 0;
+}
+
+static int cgroup_file_open(struct kernfs_open_file *of)
+{
+	struct cftype *cft = of->kn->priv;
+
+	if (cft->open)
+		return cft->open(of);
+	return 0;
+}
+
+static void cgroup_file_release(struct kernfs_open_file *of)
+{
+	struct cftype *cft = of->kn->priv;
+
+	if (cft->release)
+		cft->release(of);
+}
+
+static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
+				 size_t nbytes, loff_t off)
+{
+	struct cgroup *cgrp = of->kn->parent->priv;
+	struct cftype *cft = of->kn->priv;
+	struct cgroup_subsys_state *css;
+	int ret;
+
+	if (cft->write)
+		return cft->write(of, buf, nbytes, off);
+
+	/*
+	 * kernfs guarantees that a file isn't deleted with operations in
+	 * flight, which means that the matching css is and stays alive and
+	 * doesn't need to be pinned.  The RCU locking is not necessary
+	 * either.  It's just for the convenience of using cgroup_css().
+	 */
+	rcu_read_lock();
+	css = cgroup_css(cgrp, cft->ss);
+	rcu_read_unlock();
+
+	if (cft->write_u64) {
+		unsigned long long v;
+		ret = kstrtoull(buf, 0, &v);
+		if (!ret)
+			ret = cft->write_u64(css, cft, v);
+	} else if (cft->write_s64) {
+		long long v;
+		ret = kstrtoll(buf, 0, &v);
+		if (!ret)
+			ret = cft->write_s64(css, cft, v);
+	} else {
+		ret = -EINVAL;
+	}
+
+	return ret ?: nbytes;
+}
+
+static void *cgroup_seqfile_start(struct seq_file *seq, loff_t *ppos)
+{
+	return seq_cft(seq)->seq_start(seq, ppos);
+}
+
+static void *cgroup_seqfile_next(struct seq_file *seq, void *v, loff_t *ppos)
+{
+	return seq_cft(seq)->seq_next(seq, v, ppos);
+}
+
+static void cgroup_seqfile_stop(struct seq_file *seq, void *v)
+{
+	if (seq_cft(seq)->seq_stop)
+		seq_cft(seq)->seq_stop(seq, v);
+}
+
+static int cgroup_seqfile_show(struct seq_file *m, void *arg)
+{
+	struct cftype *cft = seq_cft(m);
+	struct cgroup_subsys_state *css = seq_css(m);
+
+	if (cft->seq_show)
+		return cft->seq_show(m, arg);
+
+	if (cft->read_u64)
+		seq_printf(m, "%llu\n", cft->read_u64(css, cft));
+	else if (cft->read_s64)
+		seq_printf(m, "%lld\n", cft->read_s64(css, cft));
+	else
+		return -EINVAL;
+	return 0;
+}
+
+static struct kernfs_ops cgroup_kf_single_ops = {
+	.atomic_write_len	= PAGE_SIZE,
+	.open			= cgroup_file_open,
+	.release		= cgroup_file_release,
+	.write			= cgroup_file_write,
+	.seq_show		= cgroup_seqfile_show,
+};
+
+static struct kernfs_ops cgroup_kf_ops = {
+	.atomic_write_len	= PAGE_SIZE,
+	.open			= cgroup_file_open,
+	.release		= cgroup_file_release,
+	.write			= cgroup_file_write,
+	.seq_start		= cgroup_seqfile_start,
+	.seq_next		= cgroup_seqfile_next,
+	.seq_stop		= cgroup_seqfile_stop,
+	.seq_show		= cgroup_seqfile_show,
+};
+
+/* set uid and gid of cgroup dirs and files to that of the creator */
+static int cgroup_kn_set_ugid(struct kernfs_node *kn)
+{
+	struct iattr iattr = { .ia_valid = ATTR_UID | ATTR_GID,
+			       .ia_uid = current_fsuid(),
+			       .ia_gid = current_fsgid(), };
+
+	if (uid_eq(iattr.ia_uid, GLOBAL_ROOT_UID) &&
+	    gid_eq(iattr.ia_gid, GLOBAL_ROOT_GID))
+		return 0;
+
+	return kernfs_setattr(kn, &iattr);
+}
+
+static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
+			   struct cftype *cft)
+{
+	char name[CGROUP_FILE_NAME_MAX];
+	struct kernfs_node *kn;
+	struct lock_class_key *key = NULL;
+	int ret;
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	key = &cft->lockdep_key;
+#endif
+	kn = __kernfs_create_file(cgrp->kn, cgroup_file_name(cgrp, cft, name),
+				  cgroup_file_mode(cft), 0, cft->kf_ops, cft,
+				  NULL, key);
+	if (IS_ERR(kn))
+		return PTR_ERR(kn);
+
+	ret = cgroup_kn_set_ugid(kn);
+	if (ret) {
+		kernfs_remove(kn);
+		return ret;
+	}
+
+	if (cft->file_offset) {
+		struct cgroup_file *cfile = (void *)css + cft->file_offset;
+
+		spin_lock_irq(&cgroup_file_kn_lock);
+		cfile->kn = kn;
+		spin_unlock_irq(&cgroup_file_kn_lock);
+	}
+
+	return 0;
+}
+
+/**
+ * cgroup_addrm_files - add or remove files to a cgroup directory
+ * @css: the target css
+ * @cgrp: the target cgroup (usually css->cgroup)
+ * @cfts: array of cftypes to be added
+ * @is_add: whether to add or remove
+ *
+ * Depending on @is_add, add or remove files defined by @cfts on @cgrp.
+ * For removals, this function never fails.
+ */
+static int cgroup_addrm_files(struct cgroup_subsys_state *css,
+			      struct cgroup *cgrp, struct cftype cfts[],
+			      bool is_add)
+{
+	struct cftype *cft, *cft_end = NULL;
+	int ret = 0;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+restart:
+	for (cft = cfts; cft != cft_end && cft->name[0] != '\0'; cft++) {
+		/* does cft->flags tell us to skip this file on @cgrp? */
+		if ((cft->flags & __CFTYPE_ONLY_ON_DFL) && !cgroup_on_dfl(cgrp))
+			continue;
+		if ((cft->flags & __CFTYPE_NOT_ON_DFL) && cgroup_on_dfl(cgrp))
+			continue;
+		if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgroup_parent(cgrp))
+			continue;
+		if ((cft->flags & CFTYPE_ONLY_ON_ROOT) && cgroup_parent(cgrp))
+			continue;
+
+		if (is_add) {
+			ret = cgroup_add_file(css, cgrp, cft);
+			if (ret) {
+				pr_warn("%s: failed to add %s, err=%d\n",
+					__func__, cft->name, ret);
+				cft_end = cft;
+				is_add = false;
+				goto restart;
+			}
+		} else {
+			cgroup_rm_file(cgrp, cft);
+		}
+	}
+	return ret;
+}
+
+static int cgroup_apply_cftypes(struct cftype *cfts, bool is_add)
+{
+	LIST_HEAD(pending);
+	struct cgroup_subsys *ss = cfts[0].ss;
+	struct cgroup *root = &ss->root->cgrp;
+	struct cgroup_subsys_state *css;
+	int ret = 0;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	/* add/rm files for all cgroups created before */
+	css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
+		struct cgroup *cgrp = css->cgroup;
+
+		if (!(css->flags & CSS_VISIBLE))
+			continue;
+
+		ret = cgroup_addrm_files(css, cgrp, cfts, is_add);
+		if (ret)
+			break;
+	}
+
+	if (is_add && !ret)
+		kernfs_activate(root->kn);
+	return ret;
+}
+
+static void cgroup_exit_cftypes(struct cftype *cfts)
+{
+	struct cftype *cft;
+
+	for (cft = cfts; cft->name[0] != '\0'; cft++) {
+		/* free copy for custom atomic_write_len, see init_cftypes() */
+		if (cft->max_write_len && cft->max_write_len != PAGE_SIZE)
+			kfree(cft->kf_ops);
+		cft->kf_ops = NULL;
+		cft->ss = NULL;
+
+		/* revert flags set by cgroup core while adding @cfts */
+		cft->flags &= ~(__CFTYPE_ONLY_ON_DFL | __CFTYPE_NOT_ON_DFL);
+	}
+}
+
+static int cgroup_init_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
+{
+	struct cftype *cft;
+
+	for (cft = cfts; cft->name[0] != '\0'; cft++) {
+		struct kernfs_ops *kf_ops;
+
+		WARN_ON(cft->ss || cft->kf_ops);
+
+		if (cft->seq_start)
+			kf_ops = &cgroup_kf_ops;
+		else
+			kf_ops = &cgroup_kf_single_ops;
+
+		/*
+		 * Ugh... if @cft wants a custom max_write_len, we need to
+		 * make a copy of kf_ops to set its atomic_write_len.
+		 */
+		if (cft->max_write_len && cft->max_write_len != PAGE_SIZE) {
+			kf_ops = kmemdup(kf_ops, sizeof(*kf_ops), GFP_KERNEL);
+			if (!kf_ops) {
+				cgroup_exit_cftypes(cfts);
+				return -ENOMEM;
+			}
+			kf_ops->atomic_write_len = cft->max_write_len;
+		}
+
+		cft->kf_ops = kf_ops;
+		cft->ss = ss;
+	}
+
+	return 0;
+}
+
+static int cgroup_rm_cftypes_locked(struct cftype *cfts)
+{
+	lockdep_assert_held(&cgroup_mutex);
+
+	if (!cfts || !cfts[0].ss)
+		return -ENOENT;
+
+	list_del(&cfts->node);
+	cgroup_apply_cftypes(cfts, false);
+	cgroup_exit_cftypes(cfts);
+	return 0;
+}
+
+/**
+ * cgroup_rm_cftypes - remove an array of cftypes from a subsystem
+ * @cfts: zero-length name terminated array of cftypes
+ *
+ * Unregister @cfts.  Files described by @cfts are removed from all
+ * existing cgroups and all future cgroups won't have them either.  This
+ * function can be called anytime whether @cfts' subsys is attached or not.
+ *
+ * Returns 0 on successful unregistration, -ENOENT if @cfts is not
+ * registered.
+ */
+int cgroup_rm_cftypes(struct cftype *cfts)
+{
+	int ret;
+
+	mutex_lock(&cgroup_mutex);
+	ret = cgroup_rm_cftypes_locked(cfts);
+	mutex_unlock(&cgroup_mutex);
+	return ret;
+}
+
+/**
+ * cgroup_add_cftypes - add an array of cftypes to a subsystem
+ * @ss: target cgroup subsystem
+ * @cfts: zero-length name terminated array of cftypes
+ *
+ * Register @cfts to @ss.  Files described by @cfts are created for all
+ * existing cgroups to which @ss is attached and all future cgroups will
+ * have them too.  This function can be called anytime whether @ss is
+ * attached or not.
+ *
+ * Returns 0 on successful registration, -errno on failure.  Note that this
+ * function currently returns 0 as long as @cfts registration is successful
+ * even if some file creation attempts on existing cgroups fail.
+ */
+static int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
+{
+	int ret;
+
+	if (!cgroup_ssid_enabled(ss->id))
+		return 0;
+
+	if (!cfts || cfts[0].name[0] == '\0')
+		return 0;
+
+	ret = cgroup_init_cftypes(ss, cfts);
+	if (ret)
+		return ret;
+
+	mutex_lock(&cgroup_mutex);
+
+	list_add_tail(&cfts->node, &ss->cfts);
+	ret = cgroup_apply_cftypes(cfts, true);
+	if (ret)
+		cgroup_rm_cftypes_locked(cfts);
+
+	mutex_unlock(&cgroup_mutex);
+	return ret;
+}
+
+/**
+ * cgroup_add_dfl_cftypes - add an array of cftypes for default hierarchy
+ * @ss: target cgroup subsystem
+ * @cfts: zero-length name terminated array of cftypes
+ *
+ * Similar to cgroup_add_cftypes() but the added files are only used for
+ * the default hierarchy.
+ */
+int cgroup_add_dfl_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
+{
+	struct cftype *cft;
+
+	for (cft = cfts; cft && cft->name[0] != '\0'; cft++)
+		cft->flags |= __CFTYPE_ONLY_ON_DFL;
+	return cgroup_add_cftypes(ss, cfts);
+}
+
+/**
+ * cgroup_add_legacy_cftypes - add an array of cftypes for legacy hierarchies
+ * @ss: target cgroup subsystem
+ * @cfts: zero-length name terminated array of cftypes
+ *
+ * Similar to cgroup_add_cftypes() but the added files are only used for
+ * the legacy hierarchies.
+ */
+int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
+{
+	struct cftype *cft;
+
+	for (cft = cfts; cft && cft->name[0] != '\0'; cft++)
+		cft->flags |= __CFTYPE_NOT_ON_DFL;
+	return cgroup_add_cftypes(ss, cfts);
+}
+
+/**
+ * cgroup_file_notify - generate a file modified event for a cgroup_file
+ * @cfile: target cgroup_file
+ *
+ * @cfile must have been obtained by setting cftype->file_offset.
+ */
+void cgroup_file_notify(struct cgroup_file *cfile)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&cgroup_file_kn_lock, flags);
+	if (cfile->kn)
+		kernfs_notify(cfile->kn);
+	spin_unlock_irqrestore(&cgroup_file_kn_lock, flags);
+}
+
+/**
+ * css_next_child - find the next child of a given css
+ * @pos: the current position (%NULL to initiate traversal)
+ * @parent: css whose children to walk
+ *
+ * This function returns the next child of @parent and should be called
+ * under either cgroup_mutex or RCU read lock.  The only requirement is
+ * that @parent and @pos are accessible.  The next sibling is guaranteed to
+ * be returned regardless of their states.
+ *
+ * If a subsystem synchronizes ->css_online() and the start of iteration, a
+ * css which finished ->css_online() is guaranteed to be visible in the
+ * future iterations and will stay visible until the last reference is put.
+ * A css which hasn't finished ->css_online() or already finished
+ * ->css_offline() may show up during traversal.  It's each subsystem's
+ * responsibility to synchronize against on/offlining.
+ */
+struct cgroup_subsys_state *css_next_child(struct cgroup_subsys_state *pos,
+					   struct cgroup_subsys_state *parent)
+{
+	struct cgroup_subsys_state *next;
+
+	cgroup_assert_mutex_or_rcu_locked();
+
+	/*
+	 * @pos could already have been unlinked from the sibling list.
+	 * Once a cgroup is removed, its ->sibling.next is no longer
+	 * updated when its next sibling changes.  CSS_RELEASED is set when
+	 * @pos is taken off list, at which time its next pointer is valid,
+	 * and, as releases are serialized, the one pointed to by the next
+	 * pointer is guaranteed to not have started release yet.  This
+	 * implies that if we observe !CSS_RELEASED on @pos in this RCU
+	 * critical section, the one pointed to by its next pointer is
+	 * guaranteed to not have finished its RCU grace period even if we
+	 * have dropped rcu_read_lock() inbetween iterations.
+	 *
+	 * If @pos has CSS_RELEASED set, its next pointer can't be
+	 * dereferenced; however, as each css is given a monotonically
+	 * increasing unique serial number and always appended to the
+	 * sibling list, the next one can be found by walking the parent's
+	 * children until the first css with higher serial number than
+	 * @pos's.  While this path can be slower, it happens iff iteration
+	 * races against release and the race window is very small.
+	 */
+	if (!pos) {
+		next = list_entry_rcu(parent->children.next, struct cgroup_subsys_state, sibling);
+	} else if (likely(!(pos->flags & CSS_RELEASED))) {
+		next = list_entry_rcu(pos->sibling.next, struct cgroup_subsys_state, sibling);
+	} else {
+		list_for_each_entry_rcu(next, &parent->children, sibling)
+			if (next->serial_nr > pos->serial_nr)
+				break;
+	}
+
+	/*
+	 * @next, if not pointing to the head, can be dereferenced and is
+	 * the next sibling.
+	 */
+	if (&next->sibling != &parent->children)
+		return next;
+	return NULL;
+}
+
+/**
+ * css_next_descendant_pre - find the next descendant for pre-order walk
+ * @pos: the current position (%NULL to initiate traversal)
+ * @root: css whose descendants to walk
+ *
+ * To be used by css_for_each_descendant_pre().  Find the next descendant
+ * to visit for pre-order traversal of @root's descendants.  @root is
+ * included in the iteration and the first node to be visited.
+ *
+ * While this function requires cgroup_mutex or RCU read locking, it
+ * doesn't require the whole traversal to be contained in a single critical
+ * section.  This function will return the correct next descendant as long
+ * as both @pos and @root are accessible and @pos is a descendant of @root.
+ *
+ * If a subsystem synchronizes ->css_online() and the start of iteration, a
+ * css which finished ->css_online() is guaranteed to be visible in the
+ * future iterations and will stay visible until the last reference is put.
+ * A css which hasn't finished ->css_online() or already finished
+ * ->css_offline() may show up during traversal.  It's each subsystem's
+ * responsibility to synchronize against on/offlining.
+ */
+struct cgroup_subsys_state *
+css_next_descendant_pre(struct cgroup_subsys_state *pos,
+			struct cgroup_subsys_state *root)
+{
+	struct cgroup_subsys_state *next;
+
+	cgroup_assert_mutex_or_rcu_locked();
+
+	/* if first iteration, visit @root */
+	if (!pos)
+		return root;
+
+	/* visit the first child if exists */
+	next = css_next_child(NULL, pos);
+	if (next)
+		return next;
+
+	/* no child, visit my or the closest ancestor's next sibling */
+	while (pos != root) {
+		next = css_next_child(pos, pos->parent);
+		if (next)
+			return next;
+		pos = pos->parent;
+	}
+
+	return NULL;
+}
+
+/**
+ * css_rightmost_descendant - return the rightmost descendant of a css
+ * @pos: css of interest
+ *
+ * Return the rightmost descendant of @pos.  If there's no descendant, @pos
+ * is returned.  This can be used during pre-order traversal to skip
+ * subtree of @pos.
+ *
+ * While this function requires cgroup_mutex or RCU read locking, it
+ * doesn't require the whole traversal to be contained in a single critical
+ * section.  This function will return the correct rightmost descendant as
+ * long as @pos is accessible.
+ */
+struct cgroup_subsys_state *
+css_rightmost_descendant(struct cgroup_subsys_state *pos)
+{
+	struct cgroup_subsys_state *last, *tmp;
+
+	cgroup_assert_mutex_or_rcu_locked();
+
+	do {
+		last = pos;
+		/* ->prev isn't RCU safe, walk ->next till the end */
+		pos = NULL;
+		css_for_each_child(tmp, last)
+			pos = tmp;
+	} while (pos);
+
+	return last;
+}
+
+static struct cgroup_subsys_state *
+css_leftmost_descendant(struct cgroup_subsys_state *pos)
+{
+	struct cgroup_subsys_state *last;
+
+	do {
+		last = pos;
+		pos = css_next_child(NULL, pos);
+	} while (pos);
+
+	return last;
+}
+
+/**
+ * css_next_descendant_post - find the next descendant for post-order walk
+ * @pos: the current position (%NULL to initiate traversal)
+ * @root: css whose descendants to walk
+ *
+ * To be used by css_for_each_descendant_post().  Find the next descendant
+ * to visit for post-order traversal of @root's descendants.  @root is
+ * included in the iteration and the last node to be visited.
+ *
+ * While this function requires cgroup_mutex or RCU read locking, it
+ * doesn't require the whole traversal to be contained in a single critical
+ * section.  This function will return the correct next descendant as long
+ * as both @pos and @cgroup are accessible and @pos is a descendant of
+ * @cgroup.
+ *
+ * If a subsystem synchronizes ->css_online() and the start of iteration, a
+ * css which finished ->css_online() is guaranteed to be visible in the
+ * future iterations and will stay visible until the last reference is put.
+ * A css which hasn't finished ->css_online() or already finished
+ * ->css_offline() may show up during traversal.  It's each subsystem's
+ * responsibility to synchronize against on/offlining.
+ */
+struct cgroup_subsys_state *
+css_next_descendant_post(struct cgroup_subsys_state *pos,
+			 struct cgroup_subsys_state *root)
+{
+	struct cgroup_subsys_state *next;
+
+	cgroup_assert_mutex_or_rcu_locked();
+
+	/* if first iteration, visit leftmost descendant which may be @root */
+	if (!pos)
+		return css_leftmost_descendant(root);
+
+	/* if we visited @root, we're done */
+	if (pos == root)
+		return NULL;
+
+	/* if there's an unvisited sibling, visit its leftmost descendant */
+	next = css_next_child(pos, pos->parent);
+	if (next)
+		return css_leftmost_descendant(next);
+
+	/* no sibling left, visit parent */
+	return pos->parent;
+}
+
+/**
+ * css_has_online_children - does a css have online children
+ * @css: the target css
+ *
+ * Returns %true if @css has any online children; otherwise, %false.  This
+ * function can be called from any context but the caller is responsible
+ * for synchronizing against on/offlining as necessary.
+ */
+bool css_has_online_children(struct cgroup_subsys_state *css)
+{
+	struct cgroup_subsys_state *child;
+	bool ret = false;
+
+	rcu_read_lock();
+	css_for_each_child(child, css) {
+		if (child->flags & CSS_ONLINE) {
+			ret = true;
+			break;
+		}
+	}
+	rcu_read_unlock();
+	return ret;
+}
+
+/**
+ * css_task_iter_advance_css_set - advance a task itererator to the next css_set
+ * @it: the iterator to advance
+ *
+ * Advance @it to the next css_set to walk.
+ */
+static void css_task_iter_advance_css_set(struct css_task_iter *it)
+{
+	struct list_head *l = it->cset_pos;
+	struct cgrp_cset_link *link;
+	struct css_set *cset;
+
+	lockdep_assert_held(&css_set_lock);
+
+	/* Advance to the next non-empty css_set */
+	do {
+		l = l->next;
+		if (l == it->cset_head) {
+			it->cset_pos = NULL;
+			it->task_pos = NULL;
+			return;
+		}
+
+		if (it->ss) {
+			cset = container_of(l, struct css_set,
+					    e_cset_node[it->ss->id]);
+		} else {
+			link = list_entry(l, struct cgrp_cset_link, cset_link);
+			cset = link->cset;
+		}
+	} while (!css_set_populated(cset));
+
+	it->cset_pos = l;
+
+	if (!list_empty(&cset->tasks))
+		it->task_pos = cset->tasks.next;
+	else
+		it->task_pos = cset->mg_tasks.next;
+
+	it->tasks_head = &cset->tasks;
+	it->mg_tasks_head = &cset->mg_tasks;
+
+	/*
+	 * We don't keep css_sets locked across iteration steps and thus
+	 * need to take steps to ensure that iteration can be resumed after
+	 * the lock is re-acquired.  Iteration is performed at two levels -
+	 * css_sets and tasks in them.
+	 *
+	 * Once created, a css_set never leaves its cgroup lists, so a
+	 * pinned css_set is guaranteed to stay put and we can resume
+	 * iteration afterwards.
+	 *
+	 * Tasks may leave @cset across iteration steps.  This is resolved
+	 * by registering each iterator with the css_set currently being
+	 * walked and making css_set_move_task() advance iterators whose
+	 * next task is leaving.
+	 */
+	if (it->cur_cset) {
+		list_del(&it->iters_node);
+		put_css_set_locked(it->cur_cset);
+	}
+	get_css_set(cset);
+	it->cur_cset = cset;
+	list_add(&it->iters_node, &cset->task_iters);
+}
+
+static void css_task_iter_advance(struct css_task_iter *it)
+{
+	struct list_head *l = it->task_pos;
+
+	lockdep_assert_held(&css_set_lock);
+	WARN_ON_ONCE(!l);
+
+	/*
+	 * Advance iterator to find next entry.  cset->tasks is consumed
+	 * first and then ->mg_tasks.  After ->mg_tasks, we move onto the
+	 * next cset.
+	 */
+	l = l->next;
+
+	if (l == it->tasks_head)
+		l = it->mg_tasks_head->next;
+
+	if (l == it->mg_tasks_head)
+		css_task_iter_advance_css_set(it);
+	else
+		it->task_pos = l;
+}
+
+/**
+ * css_task_iter_start - initiate task iteration
+ * @css: the css to walk tasks of
+ * @it: the task iterator to use
+ *
+ * Initiate iteration through the tasks of @css.  The caller can call
+ * css_task_iter_next() to walk through the tasks until the function
+ * returns NULL.  On completion of iteration, css_task_iter_end() must be
+ * called.
+ */
+void css_task_iter_start(struct cgroup_subsys_state *css,
+			 struct css_task_iter *it)
+{
+	/* no one should try to iterate before mounting cgroups */
+	WARN_ON_ONCE(!use_task_css_set_links);
+
+	memset(it, 0, sizeof(*it));
+
+	spin_lock_irq(&css_set_lock);
+
+	it->ss = css->ss;
+
+	if (it->ss)
+		it->cset_pos = &css->cgroup->e_csets[css->ss->id];
+	else
+		it->cset_pos = &css->cgroup->cset_links;
+
+	it->cset_head = it->cset_pos;
+
+	css_task_iter_advance_css_set(it);
+
+	spin_unlock_irq(&css_set_lock);
+}
+
+/**
+ * css_task_iter_next - return the next task for the iterator
+ * @it: the task iterator being iterated
+ *
+ * The "next" function for task iteration.  @it should have been
+ * initialized via css_task_iter_start().  Returns NULL when the iteration
+ * reaches the end.
+ */
+struct task_struct *css_task_iter_next(struct css_task_iter *it)
+{
+	if (it->cur_task) {
+		put_task_struct(it->cur_task);
+		it->cur_task = NULL;
+	}
+
+	spin_lock_irq(&css_set_lock);
+
+	if (it->task_pos) {
+		it->cur_task = list_entry(it->task_pos, struct task_struct,
+					  cg_list);
+		get_task_struct(it->cur_task);
+		css_task_iter_advance(it);
+	}
+
+	spin_unlock_irq(&css_set_lock);
+
+	return it->cur_task;
+}
+
+/**
+ * css_task_iter_end - finish task iteration
+ * @it: the task iterator to finish
+ *
+ * Finish task iteration started by css_task_iter_start().
+ */
+void css_task_iter_end(struct css_task_iter *it)
+{
+	if (it->cur_cset) {
+		spin_lock_irq(&css_set_lock);
+		list_del(&it->iters_node);
+		put_css_set_locked(it->cur_cset);
+		spin_unlock_irq(&css_set_lock);
+	}
+
+	if (it->cur_task)
+		put_task_struct(it->cur_task);
+}
+
+static void cgroup_procs_release(struct kernfs_open_file *of)
+{
+	if (of->priv) {
+		css_task_iter_end(of->priv);
+		kfree(of->priv);
+	}
+}
+
+static void *cgroup_procs_next(struct seq_file *s, void *v, loff_t *pos)
+{
+	struct kernfs_open_file *of = s->private;
+	struct css_task_iter *it = of->priv;
+	struct task_struct *task;
+
+	do {
+		task = css_task_iter_next(it);
+	} while (task && !thread_group_leader(task));
+
+	return task;
+}
+
+static void *cgroup_procs_start(struct seq_file *s, loff_t *pos)
+{
+	struct kernfs_open_file *of = s->private;
+	struct cgroup *cgrp = seq_css(s)->cgroup;
+	struct css_task_iter *it = of->priv;
+
+	/*
+	 * When a seq_file is seeked, it's always traversed sequentially
+	 * from position 0, so we can simply keep iterating on !0 *pos.
+	 */
+	if (!it) {
+		if (WARN_ON_ONCE((*pos)++))
+			return ERR_PTR(-EINVAL);
+
+		it = kzalloc(sizeof(*it), GFP_KERNEL);
+		if (!it)
+			return ERR_PTR(-ENOMEM);
+		of->priv = it;
+		css_task_iter_start(&cgrp->self, it);
+	} else if (!(*pos)++) {
+		css_task_iter_end(it);
+		css_task_iter_start(&cgrp->self, it);
+	}
+
+	return cgroup_procs_next(s, NULL, NULL);
+}
+
+static int cgroup_procs_show(struct seq_file *s, void *v)
+{
+	seq_printf(s, "%d\n", task_tgid_vnr(v));
+	return 0;
+}
+
+/* cgroup core interface files for the default hierarchy */
+static struct cftype cgroup_base_files[] = {
+	{
+		.name = "cgroup.procs",
+		.file_offset = offsetof(struct cgroup, procs_file),
+		.release = cgroup_procs_release,
+		.seq_start = cgroup_procs_start,
+		.seq_next = cgroup_procs_next,
+		.seq_show = cgroup_procs_show,
+		.write = cgroup_procs_write,
+	},
+	{
+		.name = "cgroup.controllers",
+		.seq_show = cgroup_controllers_show,
+	},
+	{
+		.name = "cgroup.subtree_control",
+		.seq_show = cgroup_subtree_control_show,
+		.write = cgroup_subtree_control_write,
+	},
+	{
+		.name = "cgroup.events",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.file_offset = offsetof(struct cgroup, events_file),
+		.seq_show = cgroup_events_show,
+	},
+	{ }	/* terminate */
+};
+
+/*
+ * css destruction is four-stage process.
+ *
+ * 1. Destruction starts.  Killing of the percpu_ref is initiated.
+ *    Implemented in kill_css().
+ *
+ * 2. When the percpu_ref is confirmed to be visible as killed on all CPUs
+ *    and thus css_tryget_online() is guaranteed to fail, the css can be
+ *    offlined by invoking offline_css().  After offlining, the base ref is
+ *    put.  Implemented in css_killed_work_fn().
+ *
+ * 3. When the percpu_ref reaches zero, the only possible remaining
+ *    accessors are inside RCU read sections.  css_release() schedules the
+ *    RCU callback.
+ *
+ * 4. After the grace period, the css can be freed.  Implemented in
+ *    css_free_work_fn().
+ *
+ * It is actually hairier because both step 2 and 4 require process context
+ * and thus involve punting to css->destroy_work adding two additional
+ * steps to the already complex sequence.
+ */
+static void css_free_work_fn(struct work_struct *work)
+{
+	struct cgroup_subsys_state *css =
+		container_of(work, struct cgroup_subsys_state, destroy_work);
+	struct cgroup_subsys *ss = css->ss;
+	struct cgroup *cgrp = css->cgroup;
+
+	percpu_ref_exit(&css->refcnt);
+
+	if (ss) {
+		/* css free path */
+		struct cgroup_subsys_state *parent = css->parent;
+		int id = css->id;
+
+		ss->css_free(css);
+		cgroup_idr_remove(&ss->css_idr, id);
+		cgroup_put(cgrp);
+
+		if (parent)
+			css_put(parent);
+	} else {
+		/* cgroup free path */
+		atomic_dec(&cgrp->root->nr_cgrps);
+		cgroup1_pidlist_destroy_all(cgrp);
+		cancel_work_sync(&cgrp->release_agent_work);
+
+		if (cgroup_parent(cgrp)) {
+			/*
+			 * We get a ref to the parent, and put the ref when
+			 * this cgroup is being freed, so it's guaranteed
+			 * that the parent won't be destroyed before its
+			 * children.
+			 */
+			cgroup_put(cgroup_parent(cgrp));
+			kernfs_put(cgrp->kn);
+			kfree(cgrp);
+		} else {
+			/*
+			 * This is root cgroup's refcnt reaching zero,
+			 * which indicates that the root should be
+			 * released.
+			 */
+			cgroup_destroy_root(cgrp->root);
+		}
+	}
+}
+
+static void css_free_rcu_fn(struct rcu_head *rcu_head)
+{
+	struct cgroup_subsys_state *css =
+		container_of(rcu_head, struct cgroup_subsys_state, rcu_head);
+
+	INIT_WORK(&css->destroy_work, css_free_work_fn);
+	queue_work(cgroup_destroy_wq, &css->destroy_work);
+}
+
+static void css_release_work_fn(struct work_struct *work)
+{
+	struct cgroup_subsys_state *css =
+		container_of(work, struct cgroup_subsys_state, destroy_work);
+	struct cgroup_subsys *ss = css->ss;
+	struct cgroup *cgrp = css->cgroup;
+
+	mutex_lock(&cgroup_mutex);
+
+	css->flags |= CSS_RELEASED;
+	list_del_rcu(&css->sibling);
+
+	if (ss) {
+		/* css release path */
+		cgroup_idr_replace(&ss->css_idr, NULL, css->id);
+		if (ss->css_released)
+			ss->css_released(css);
+	} else {
+		/* cgroup release path */
+		trace_cgroup_release(cgrp);
+
+		cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
+		cgrp->id = -1;
+
+		/*
+		 * There are two control paths which try to determine
+		 * cgroup from dentry without going through kernfs -
+		 * cgroupstats_build() and css_tryget_online_from_dir().
+		 * Those are supported by RCU protecting clearing of
+		 * cgrp->kn->priv backpointer.
+		 */
+		if (cgrp->kn)
+			RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv,
+					 NULL);
+
+		cgroup_bpf_put(cgrp);
+	}
+
+	mutex_unlock(&cgroup_mutex);
+
+	call_rcu(&css->rcu_head, css_free_rcu_fn);
+}
+
+static void css_release(struct percpu_ref *ref)
+{
+	struct cgroup_subsys_state *css =
+		container_of(ref, struct cgroup_subsys_state, refcnt);
+
+	INIT_WORK(&css->destroy_work, css_release_work_fn);
+	queue_work(cgroup_destroy_wq, &css->destroy_work);
+}
+
+static void init_and_link_css(struct cgroup_subsys_state *css,
+			      struct cgroup_subsys *ss, struct cgroup *cgrp)
+{
+	lockdep_assert_held(&cgroup_mutex);
+
+	cgroup_get(cgrp);
+
+	memset(css, 0, sizeof(*css));
+	css->cgroup = cgrp;
+	css->ss = ss;
+	css->id = -1;
+	INIT_LIST_HEAD(&css->sibling);
+	INIT_LIST_HEAD(&css->children);
+	css->serial_nr = css_serial_nr_next++;
+	atomic_set(&css->online_cnt, 0);
+
+	if (cgroup_parent(cgrp)) {
+		css->parent = cgroup_css(cgroup_parent(cgrp), ss);
+		css_get(css->parent);
+	}
+
+	BUG_ON(cgroup_css(cgrp, ss));
+}
+
+/* invoke ->css_online() on a new CSS and mark it online if successful */
+static int online_css(struct cgroup_subsys_state *css)
+{
+	struct cgroup_subsys *ss = css->ss;
+	int ret = 0;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	if (ss->css_online)
+		ret = ss->css_online(css);
+	if (!ret) {
+		css->flags |= CSS_ONLINE;
+		rcu_assign_pointer(css->cgroup->subsys[ss->id], css);
+
+		atomic_inc(&css->online_cnt);
+		if (css->parent)
+			atomic_inc(&css->parent->online_cnt);
+	}
+	return ret;
+}
+
+/* if the CSS is online, invoke ->css_offline() on it and mark it offline */
+static void offline_css(struct cgroup_subsys_state *css)
+{
+	struct cgroup_subsys *ss = css->ss;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	if (!(css->flags & CSS_ONLINE))
+		return;
+
+	if (ss->css_reset)
+		ss->css_reset(css);
+
+	if (ss->css_offline)
+		ss->css_offline(css);
+
+	css->flags &= ~CSS_ONLINE;
+	RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL);
+
+	wake_up_all(&css->cgroup->offline_waitq);
+}
+
+/**
+ * css_create - create a cgroup_subsys_state
+ * @cgrp: the cgroup new css will be associated with
+ * @ss: the subsys of new css
+ *
+ * Create a new css associated with @cgrp - @ss pair.  On success, the new
+ * css is online and installed in @cgrp.  This function doesn't create the
+ * interface files.  Returns 0 on success, -errno on failure.
+ */
+static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
+					      struct cgroup_subsys *ss)
+{
+	struct cgroup *parent = cgroup_parent(cgrp);
+	struct cgroup_subsys_state *parent_css = cgroup_css(parent, ss);
+	struct cgroup_subsys_state *css;
+	int err;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	css = ss->css_alloc(parent_css);
+	if (!css)
+		css = ERR_PTR(-ENOMEM);
+	if (IS_ERR(css))
+		return css;
+
+	init_and_link_css(css, ss, cgrp);
+
+	err = percpu_ref_init(&css->refcnt, css_release, 0, GFP_KERNEL);
+	if (err)
+		goto err_free_css;
+
+	err = cgroup_idr_alloc(&ss->css_idr, NULL, 2, 0, GFP_KERNEL);
+	if (err < 0)
+		goto err_free_css;
+	css->id = err;
+
+	/* @css is ready to be brought online now, make it visible */
+	list_add_tail_rcu(&css->sibling, &parent_css->children);
+	cgroup_idr_replace(&ss->css_idr, css, css->id);
+
+	err = online_css(css);
+	if (err)
+		goto err_list_del;
+
+	if (ss->broken_hierarchy && !ss->warned_broken_hierarchy &&
+	    cgroup_parent(parent)) {
+		pr_warn("%s (%d) created nested cgroup for controller \"%s\" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.\n",
+			current->comm, current->pid, ss->name);
+		if (!strcmp(ss->name, "memory"))
+			pr_warn("\"memory\" requires setting use_hierarchy to 1 on the root\n");
+		ss->warned_broken_hierarchy = true;
+	}
+
+	return css;
+
+err_list_del:
+	list_del_rcu(&css->sibling);
+err_free_css:
+	call_rcu(&css->rcu_head, css_free_rcu_fn);
+	return ERR_PTR(err);
+}
+
+/*
+ * The returned cgroup is fully initialized including its control mask, but
+ * it isn't associated with its kernfs_node and doesn't have the control
+ * mask applied.
+ */
+static struct cgroup *cgroup_create(struct cgroup *parent)
+{
+	struct cgroup_root *root = parent->root;
+	struct cgroup *cgrp, *tcgrp;
+	int level = parent->level + 1;
+	int ret;
+
+	/* allocate the cgroup and its ID, 0 is reserved for the root */
+	cgrp = kzalloc(sizeof(*cgrp) +
+		       sizeof(cgrp->ancestor_ids[0]) * (level + 1), GFP_KERNEL);
+	if (!cgrp)
+		return ERR_PTR(-ENOMEM);
+
+	ret = percpu_ref_init(&cgrp->self.refcnt, css_release, 0, GFP_KERNEL);
+	if (ret)
+		goto out_free_cgrp;
+
+	/*
+	 * Temporarily set the pointer to NULL, so idr_find() won't return
+	 * a half-baked cgroup.
+	 */
+	cgrp->id = cgroup_idr_alloc(&root->cgroup_idr, NULL, 2, 0, GFP_KERNEL);
+	if (cgrp->id < 0) {
+		ret = -ENOMEM;
+		goto out_cancel_ref;
+	}
+
+	init_cgroup_housekeeping(cgrp);
+
+	cgrp->self.parent = &parent->self;
+	cgrp->root = root;
+	cgrp->level = level;
+
+	for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp))
+		cgrp->ancestor_ids[tcgrp->level] = tcgrp->id;
+
+	if (notify_on_release(parent))
+		set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
+
+	if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &parent->flags))
+		set_bit(CGRP_CPUSET_CLONE_CHILDREN, &cgrp->flags);
+
+	cgrp->self.serial_nr = css_serial_nr_next++;
+
+	/* allocation complete, commit to creation */
+	list_add_tail_rcu(&cgrp->self.sibling, &cgroup_parent(cgrp)->self.children);
+	atomic_inc(&root->nr_cgrps);
+	cgroup_get(parent);
+
+	/*
+	 * @cgrp is now fully operational.  If something fails after this
+	 * point, it'll be released via the normal destruction path.
+	 */
+	cgroup_idr_replace(&root->cgroup_idr, cgrp, cgrp->id);
+
+	/*
+	 * On the default hierarchy, a child doesn't automatically inherit
+	 * subtree_control from the parent.  Each is configured manually.
+	 */
+	if (!cgroup_on_dfl(cgrp))
+		cgrp->subtree_control = cgroup_control(cgrp);
+
+	if (parent)
+		cgroup_bpf_inherit(cgrp, parent);
+
+	cgroup_propagate_control(cgrp);
+
+	return cgrp;
+
+out_cancel_ref:
+	percpu_ref_exit(&cgrp->self.refcnt);
+out_free_cgrp:
+	kfree(cgrp);
+	return ERR_PTR(ret);
+}
+
+int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
+{
+	struct cgroup *parent, *cgrp;
+	struct kernfs_node *kn;
+	int ret;
+
+	/* do not accept '\n' to prevent making /proc/<pid>/cgroup unparsable */
+	if (strchr(name, '\n'))
+		return -EINVAL;
+
+	parent = cgroup_kn_lock_live(parent_kn, false);
+	if (!parent)
+		return -ENODEV;
+
+	cgrp = cgroup_create(parent);
+	if (IS_ERR(cgrp)) {
+		ret = PTR_ERR(cgrp);
+		goto out_unlock;
+	}
+
+	/* create the directory */
+	kn = kernfs_create_dir(parent->kn, name, mode, cgrp);
+	if (IS_ERR(kn)) {
+		ret = PTR_ERR(kn);
+		goto out_destroy;
+	}
+	cgrp->kn = kn;
+
+	/*
+	 * This extra ref will be put in cgroup_free_fn() and guarantees
+	 * that @cgrp->kn is always accessible.
+	 */
+	kernfs_get(kn);
+
+	ret = cgroup_kn_set_ugid(kn);
+	if (ret)
+		goto out_destroy;
+
+	ret = css_populate_dir(&cgrp->self);
+	if (ret)
+		goto out_destroy;
+
+	ret = cgroup_apply_control_enable(cgrp);
+	if (ret)
+		goto out_destroy;
+
+	trace_cgroup_mkdir(cgrp);
+
+	/* let's create and online css's */
+	kernfs_activate(kn);
+
+	ret = 0;
+	goto out_unlock;
+
+out_destroy:
+	cgroup_destroy_locked(cgrp);
+out_unlock:
+	cgroup_kn_unlock(parent_kn);
+	return ret;
+}
+
+/*
+ * This is called when the refcnt of a css is confirmed to be killed.
+ * css_tryget_online() is now guaranteed to fail.  Tell the subsystem to
+ * initate destruction and put the css ref from kill_css().
+ */
+static void css_killed_work_fn(struct work_struct *work)
+{
+	struct cgroup_subsys_state *css =
+		container_of(work, struct cgroup_subsys_state, destroy_work);
+
+	mutex_lock(&cgroup_mutex);
+
+	do {
+		offline_css(css);
+		css_put(css);
+		/* @css can't go away while we're holding cgroup_mutex */
+		css = css->parent;
+	} while (css && atomic_dec_and_test(&css->online_cnt));
+
+	mutex_unlock(&cgroup_mutex);
+}
+
+/* css kill confirmation processing requires process context, bounce */
+static void css_killed_ref_fn(struct percpu_ref *ref)
+{
+	struct cgroup_subsys_state *css =
+		container_of(ref, struct cgroup_subsys_state, refcnt);
+
+	if (atomic_dec_and_test(&css->online_cnt)) {
+		INIT_WORK(&css->destroy_work, css_killed_work_fn);
+		queue_work(cgroup_destroy_wq, &css->destroy_work);
+	}
+}
+
+/**
+ * kill_css - destroy a css
+ * @css: css to destroy
+ *
+ * This function initiates destruction of @css by removing cgroup interface
+ * files and putting its base reference.  ->css_offline() will be invoked
+ * asynchronously once css_tryget_online() is guaranteed to fail and when
+ * the reference count reaches zero, @css will be released.
+ */
+static void kill_css(struct cgroup_subsys_state *css)
+{
+	lockdep_assert_held(&cgroup_mutex);
+
+	/*
+	 * This must happen before css is disassociated with its cgroup.
+	 * See seq_css() for details.
+	 */
+	css_clear_dir(css);
+
+	/*
+	 * Killing would put the base ref, but we need to keep it alive
+	 * until after ->css_offline().
+	 */
+	css_get(css);
+
+	/*
+	 * cgroup core guarantees that, by the time ->css_offline() is
+	 * invoked, no new css reference will be given out via
+	 * css_tryget_online().  We can't simply call percpu_ref_kill() and
+	 * proceed to offlining css's because percpu_ref_kill() doesn't
+	 * guarantee that the ref is seen as killed on all CPUs on return.
+	 *
+	 * Use percpu_ref_kill_and_confirm() to get notifications as each
+	 * css is confirmed to be seen as killed on all CPUs.
+	 */
+	percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn);
+}
+
+/**
+ * cgroup_destroy_locked - the first stage of cgroup destruction
+ * @cgrp: cgroup to be destroyed
+ *
+ * css's make use of percpu refcnts whose killing latency shouldn't be
+ * exposed to userland and are RCU protected.  Also, cgroup core needs to
+ * guarantee that css_tryget_online() won't succeed by the time
+ * ->css_offline() is invoked.  To satisfy all the requirements,
+ * destruction is implemented in the following two steps.
+ *
+ * s1. Verify @cgrp can be destroyed and mark it dying.  Remove all
+ *     userland visible parts and start killing the percpu refcnts of
+ *     css's.  Set up so that the next stage will be kicked off once all
+ *     the percpu refcnts are confirmed to be killed.
+ *
+ * s2. Invoke ->css_offline(), mark the cgroup dead and proceed with the
+ *     rest of destruction.  Once all cgroup references are gone, the
+ *     cgroup is RCU-freed.
+ *
+ * This function implements s1.  After this step, @cgrp is gone as far as
+ * the userland is concerned and a new cgroup with the same name may be
+ * created.  As cgroup doesn't care about the names internally, this
+ * doesn't cause any problem.
+ */
+static int cgroup_destroy_locked(struct cgroup *cgrp)
+	__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
+{
+	struct cgroup_subsys_state *css;
+	struct cgrp_cset_link *link;
+	int ssid;
+
+	lockdep_assert_held(&cgroup_mutex);
+
+	/*
+	 * Only migration can raise populated from zero and we're already
+	 * holding cgroup_mutex.
+	 */
+	if (cgroup_is_populated(cgrp))
+		return -EBUSY;
+
+	/*
+	 * Make sure there's no live children.  We can't test emptiness of
+	 * ->self.children as dead children linger on it while being
+	 * drained; otherwise, "rmdir parent/child parent" may fail.
+	 */
+	if (css_has_online_children(&cgrp->self))
+		return -EBUSY;
+
+	/*
+	 * Mark @cgrp and the associated csets dead.  The former prevents
+	 * further task migration and child creation by disabling
+	 * cgroup_lock_live_group().  The latter makes the csets ignored by
+	 * the migration path.
+	 */
+	cgrp->self.flags &= ~CSS_ONLINE;
+
+	spin_lock_irq(&css_set_lock);
+	list_for_each_entry(link, &cgrp->cset_links, cset_link)
+		link->cset->dead = true;
+	spin_unlock_irq(&css_set_lock);
+
+	/* initiate massacre of all css's */
+	for_each_css(css, ssid, cgrp)
+		kill_css(css);
+
+	/*
+	 * Remove @cgrp directory along with the base files.  @cgrp has an
+	 * extra ref on its kn.
+	 */
+	kernfs_remove(cgrp->kn);
+
+	cgroup1_check_for_release(cgroup_parent(cgrp));
+
+	/* put the base reference */
+	percpu_ref_kill(&cgrp->self.refcnt);
+
+	return 0;
+};
+
+int cgroup_rmdir(struct kernfs_node *kn)
+{
+	struct cgroup *cgrp;
+	int ret = 0;
+
+	cgrp = cgroup_kn_lock_live(kn, false);
+	if (!cgrp)
+		return 0;
+
+	ret = cgroup_destroy_locked(cgrp);
+
+	if (!ret)
+		trace_cgroup_rmdir(cgrp);
+
+	cgroup_kn_unlock(kn);
+	return ret;
+}
+
+static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
+	.remount_fs		= cgroup_remount,
+	.mkdir			= cgroup_mkdir,
+	.rmdir			= cgroup_rmdir,
+	.show_path		= cgroup_show_path,
+};
+
+static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
+{
+	struct cgroup_subsys_state *css;
+
+	pr_debug("Initializing cgroup subsys %s\n", ss->name);
+
+	mutex_lock(&cgroup_mutex);
+
+	idr_init(&ss->css_idr);
+	INIT_LIST_HEAD(&ss->cfts);
+
+	/* Create the root cgroup state for this subsystem */
+	ss->root = &cgrp_dfl_root;
+	css = ss->css_alloc(cgroup_css(&cgrp_dfl_root.cgrp, ss));
+	/* We don't handle early failures gracefully */
+	BUG_ON(IS_ERR(css));
+	init_and_link_css(css, ss, &cgrp_dfl_root.cgrp);
+
+	/*
+	 * Root csses are never destroyed and we can't initialize
+	 * percpu_ref during early init.  Disable refcnting.
+	 */
+	css->flags |= CSS_NO_REF;
+
+	if (early) {
+		/* allocation can't be done safely during early init */
+		css->id = 1;
+	} else {
+		css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2, GFP_KERNEL);
+		BUG_ON(css->id < 0);
+	}
+
+	/* Update the init_css_set to contain a subsys
+	 * pointer to this state - since the subsystem is
+	 * newly registered, all tasks and hence the
+	 * init_css_set is in the subsystem's root cgroup. */
+	init_css_set.subsys[ss->id] = css;
+
+	have_fork_callback |= (bool)ss->fork << ss->id;
+	have_exit_callback |= (bool)ss->exit << ss->id;
+	have_free_callback |= (bool)ss->free << ss->id;
+	have_canfork_callback |= (bool)ss->can_fork << ss->id;
+
+	/* At system boot, before all subsystems have been
+	 * registered, no tasks have been forked, so we don't
+	 * need to invoke fork callbacks here. */
+	BUG_ON(!list_empty(&init_task.tasks));
+
+	BUG_ON(online_css(css));
+
+	mutex_unlock(&cgroup_mutex);
+}
+
+/**
+ * cgroup_init_early - cgroup initialization at system boot
+ *
+ * Initialize cgroups at system boot, and initialize any
+ * subsystems that request early init.
+ */
+int __init cgroup_init_early(void)
+{
+	static struct cgroup_sb_opts __initdata opts;
+	struct cgroup_subsys *ss;
+	int i;
+
+	init_cgroup_root(&cgrp_dfl_root, &opts);
+	cgrp_dfl_root.cgrp.self.flags |= CSS_NO_REF;
+
+	RCU_INIT_POINTER(init_task.cgroups, &init_css_set);
+
+	for_each_subsys(ss, i) {
+		WARN(!ss->css_alloc || !ss->css_free || ss->name || ss->id,
+		     "invalid cgroup_subsys %d:%s css_alloc=%p css_free=%p id:name=%d:%s\n",
+		     i, cgroup_subsys_name[i], ss->css_alloc, ss->css_free,
+		     ss->id, ss->name);
+		WARN(strlen(cgroup_subsys_name[i]) > MAX_CGROUP_TYPE_NAMELEN,
+		     "cgroup_subsys_name %s too long\n", cgroup_subsys_name[i]);
+
+		ss->id = i;
+		ss->name = cgroup_subsys_name[i];
+		if (!ss->legacy_name)
+			ss->legacy_name = cgroup_subsys_name[i];
+
+		if (ss->early_init)
+			cgroup_init_subsys(ss, true);
+	}
+	return 0;
+}
+
+static u16 cgroup_disable_mask __initdata;
+
+/**
+ * cgroup_init - cgroup initialization
+ *
+ * Register cgroup filesystem and /proc file, and initialize
+ * any subsystems that didn't request early init.
+ */
+int __init cgroup_init(void)
+{
+	struct cgroup_subsys *ss;
+	int ssid;
+
+	BUILD_BUG_ON(CGROUP_SUBSYS_COUNT > 16);
+	BUG_ON(percpu_init_rwsem(&cgroup_threadgroup_rwsem));
+	BUG_ON(cgroup_init_cftypes(NULL, cgroup_base_files));
+	BUG_ON(cgroup_init_cftypes(NULL, cgroup1_base_files));
+
+	/*
+	 * The latency of the synchronize_sched() is too high for cgroups,
+	 * avoid it at the cost of forcing all readers into the slow path.
+	 */
+	rcu_sync_enter_start(&cgroup_threadgroup_rwsem.rss);
+
+	get_user_ns(init_cgroup_ns.user_ns);
+
+	mutex_lock(&cgroup_mutex);
+
+	/*
+	 * Add init_css_set to the hash table so that dfl_root can link to
+	 * it during init.
+	 */
+	hash_add(css_set_table, &init_css_set.hlist,
+		 css_set_hash(init_css_set.subsys));
+
+	BUG_ON(cgroup_setup_root(&cgrp_dfl_root, 0));
+
+	mutex_unlock(&cgroup_mutex);
+
+	for_each_subsys(ss, ssid) {
+		if (ss->early_init) {
+			struct cgroup_subsys_state *css =
+				init_css_set.subsys[ss->id];
+
+			css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2,
+						   GFP_KERNEL);
+			BUG_ON(css->id < 0);
+		} else {
+			cgroup_init_subsys(ss, false);
+		}
+
+		list_add_tail(&init_css_set.e_cset_node[ssid],
+			      &cgrp_dfl_root.cgrp.e_csets[ssid]);
+
+		/*
+		 * Setting dfl_root subsys_mask needs to consider the
+		 * disabled flag and cftype registration needs kmalloc,
+		 * both of which aren't available during early_init.
+		 */
+		if (cgroup_disable_mask & (1 << ssid)) {
+			static_branch_disable(cgroup_subsys_enabled_key[ssid]);
+			printk(KERN_INFO "Disabling %s control group subsystem\n",
+			       ss->name);
+			continue;
+		}
+
+		if (cgroup1_ssid_disabled(ssid))
+			printk(KERN_INFO "Disabling %s control group subsystem in v1 mounts\n",
+			       ss->name);
+
+		cgrp_dfl_root.subsys_mask |= 1 << ss->id;
+
+		if (ss->implicit_on_dfl)
+			cgrp_dfl_implicit_ss_mask |= 1 << ss->id;
+		else if (!ss->dfl_cftypes)
+			cgrp_dfl_inhibit_ss_mask |= 1 << ss->id;
+
+		if (ss->dfl_cftypes == ss->legacy_cftypes) {
+			WARN_ON(cgroup_add_cftypes(ss, ss->dfl_cftypes));
+		} else {
+			WARN_ON(cgroup_add_dfl_cftypes(ss, ss->dfl_cftypes));
+			WARN_ON(cgroup_add_legacy_cftypes(ss, ss->legacy_cftypes));
+		}
+
+		if (ss->bind)
+			ss->bind(init_css_set.subsys[ssid]);
+	}
+
+	/* init_css_set.subsys[] has been updated, re-hash */
+	hash_del(&init_css_set.hlist);
+	hash_add(css_set_table, &init_css_set.hlist,
+		 css_set_hash(init_css_set.subsys));
+
+	WARN_ON(sysfs_create_mount_point(fs_kobj, "cgroup"));
+	WARN_ON(register_filesystem(&cgroup_fs_type));
+	WARN_ON(register_filesystem(&cgroup2_fs_type));
+	WARN_ON(!proc_create("cgroups", 0, NULL, &proc_cgroupstats_operations));
+
+	return 0;
+}
+
+static int __init cgroup_wq_init(void)
+{
+	/*
+	 * There isn't much point in executing destruction path in
+	 * parallel.  Good chunk is serialized with cgroup_mutex anyway.
+	 * Use 1 for @max_active.
+	 *
+	 * We would prefer to do this in cgroup_init() above, but that
+	 * is called before init_workqueues(): so leave this until after.
+	 */
+	cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
+	BUG_ON(!cgroup_destroy_wq);
+	return 0;
+}
+core_initcall(cgroup_wq_init);
+
+/*
+ * proc_cgroup_show()
+ *  - Print task's cgroup paths into seq_file, one line for each hierarchy
+ *  - Used for /proc/<pid>/cgroup.
+ */
+int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
+		     struct pid *pid, struct task_struct *tsk)
+{
+	char *buf;
+	int retval;
+	struct cgroup_root *root;
+
+	retval = -ENOMEM;
+	buf = kmalloc(PATH_MAX, GFP_KERNEL);
+	if (!buf)
+		goto out;
+
+	mutex_lock(&cgroup_mutex);
+	spin_lock_irq(&css_set_lock);
+
+	for_each_root(root) {
+		struct cgroup_subsys *ss;
+		struct cgroup *cgrp;
+		int ssid, count = 0;
+
+		if (root == &cgrp_dfl_root && !cgrp_dfl_visible)
+			continue;
+
+		seq_printf(m, "%d:", root->hierarchy_id);
+		if (root != &cgrp_dfl_root)
+			for_each_subsys(ss, ssid)
+				if (root->subsys_mask & (1 << ssid))
+					seq_printf(m, "%s%s", count++ ? "," : "",
+						   ss->legacy_name);
+		if (strlen(root->name))
+			seq_printf(m, "%sname=%s", count ? "," : "",
+				   root->name);
+		seq_putc(m, ':');
+
+		cgrp = task_cgroup_from_root(tsk, root);
+
+		/*
+		 * On traditional hierarchies, all zombie tasks show up as
+		 * belonging to the root cgroup.  On the default hierarchy,
+		 * while a zombie doesn't show up in "cgroup.procs" and
+		 * thus can't be migrated, its /proc/PID/cgroup keeps
+		 * reporting the cgroup it belonged to before exiting.  If
+		 * the cgroup is removed before the zombie is reaped,
+		 * " (deleted)" is appended to the cgroup path.
+		 */
+		if (cgroup_on_dfl(cgrp) || !(tsk->flags & PF_EXITING)) {
+			retval = cgroup_path_ns_locked(cgrp, buf, PATH_MAX,
+						current->nsproxy->cgroup_ns);
+			if (retval >= PATH_MAX)
+				retval = -ENAMETOOLONG;
+			if (retval < 0)
+				goto out_unlock;
+
+			seq_puts(m, buf);
+		} else {
+			seq_puts(m, "/");
+		}
+
+		if (cgroup_on_dfl(cgrp) && cgroup_is_dead(cgrp))
+			seq_puts(m, " (deleted)\n");
+		else
+			seq_putc(m, '\n');
+	}
+
+	retval = 0;
+out_unlock:
+	spin_unlock_irq(&css_set_lock);
+	mutex_unlock(&cgroup_mutex);
+	kfree(buf);
+out:
+	return retval;
+}
+
+/**
+ * cgroup_fork - initialize cgroup related fields during copy_process()
+ * @child: pointer to task_struct of forking parent process.
+ *
+ * A task is associated with the init_css_set until cgroup_post_fork()
+ * attaches it to the parent's css_set.  Empty cg_list indicates that
+ * @child isn't holding reference to its css_set.
+ */
+void cgroup_fork(struct task_struct *child)
+{
+	RCU_INIT_POINTER(child->cgroups, &init_css_set);
+	INIT_LIST_HEAD(&child->cg_list);
+}
+
+/**
+ * cgroup_can_fork - called on a new task before the process is exposed
+ * @child: the task in question.
+ *
+ * This calls the subsystem can_fork() callbacks. If the can_fork() callback
+ * returns an error, the fork aborts with that error code. This allows for
+ * a cgroup subsystem to conditionally allow or deny new forks.
+ */
+int cgroup_can_fork(struct task_struct *child)
+{
+	struct cgroup_subsys *ss;
+	int i, j, ret;
+
+	do_each_subsys_mask(ss, i, have_canfork_callback) {
+		ret = ss->can_fork(child);
+		if (ret)
+			goto out_revert;
+	} while_each_subsys_mask();
+
+	return 0;
+
+out_revert:
+	for_each_subsys(ss, j) {
+		if (j >= i)
+			break;
+		if (ss->cancel_fork)
+			ss->cancel_fork(child);
+	}
+
+	return ret;
+}
+
+/**
+ * cgroup_cancel_fork - called if a fork failed after cgroup_can_fork()
+ * @child: the task in question
+ *
+ * This calls the cancel_fork() callbacks if a fork failed *after*
+ * cgroup_can_fork() succeded.
+ */
+void cgroup_cancel_fork(struct task_struct *child)
+{
+	struct cgroup_subsys *ss;
+	int i;
+
+	for_each_subsys(ss, i)
+		if (ss->cancel_fork)
+			ss->cancel_fork(child);
+}
+
+/**
+ * cgroup_post_fork - called on a new task after adding it to the task list
+ * @child: the task in question
+ *
+ * Adds the task to the list running through its css_set if necessary and
+ * call the subsystem fork() callbacks.  Has to be after the task is
+ * visible on the task list in case we race with the first call to
+ * cgroup_task_iter_start() - to guarantee that the new task ends up on its
+ * list.
+ */
+void cgroup_post_fork(struct task_struct *child)
+{
+	struct cgroup_subsys *ss;
+	int i;
+
+	/*
+	 * This may race against cgroup_enable_task_cg_lists().  As that
+	 * function sets use_task_css_set_links before grabbing
+	 * tasklist_lock and we just went through tasklist_lock to add
+	 * @child, it's guaranteed that either we see the set
+	 * use_task_css_set_links or cgroup_enable_task_cg_lists() sees
+	 * @child during its iteration.
+	 *
+	 * If we won the race, @child is associated with %current's
+	 * css_set.  Grabbing css_set_lock guarantees both that the
+	 * association is stable, and, on completion of the parent's
+	 * migration, @child is visible in the source of migration or
+	 * already in the destination cgroup.  This guarantee is necessary
+	 * when implementing operations which need to migrate all tasks of
+	 * a cgroup to another.
+	 *
+	 * Note that if we lose to cgroup_enable_task_cg_lists(), @child
+	 * will remain in init_css_set.  This is safe because all tasks are
+	 * in the init_css_set before cg_links is enabled and there's no
+	 * operation which transfers all tasks out of init_css_set.
+	 */
+	if (use_task_css_set_links) {
+		struct css_set *cset;
+
+		spin_lock_irq(&css_set_lock);
+		cset = task_css_set(current);
+		if (list_empty(&child->cg_list)) {
+			get_css_set(cset);
+			css_set_move_task(child, NULL, cset, false);
+		}
+		spin_unlock_irq(&css_set_lock);
+	}
+
+	/*
+	 * Call ss->fork().  This must happen after @child is linked on
+	 * css_set; otherwise, @child might change state between ->fork()
+	 * and addition to css_set.
+	 */
+	do_each_subsys_mask(ss, i, have_fork_callback) {
+		ss->fork(child);
+	} while_each_subsys_mask();
+}
+
+/**
+ * cgroup_exit - detach cgroup from exiting task
+ * @tsk: pointer to task_struct of exiting process
+ *
+ * Description: Detach cgroup from @tsk and release it.
+ *
+ * Note that cgroups marked notify_on_release force every task in
+ * them to take the global cgroup_mutex mutex when exiting.
+ * This could impact scaling on very large systems.  Be reluctant to
+ * use notify_on_release cgroups where very high task exit scaling
+ * is required on large systems.
+ *
+ * We set the exiting tasks cgroup to the root cgroup (top_cgroup).  We
+ * call cgroup_exit() while the task is still competent to handle
+ * notify_on_release(), then leave the task attached to the root cgroup in
+ * each hierarchy for the remainder of its exit.  No need to bother with
+ * init_css_set refcnting.  init_css_set never goes away and we can't race
+ * with migration path - PF_EXITING is visible to migration path.
+ */
+void cgroup_exit(struct task_struct *tsk)
+{
+	struct cgroup_subsys *ss;
+	struct css_set *cset;
+	int i;
+
+	/*
+	 * Unlink from @tsk from its css_set.  As migration path can't race
+	 * with us, we can check css_set and cg_list without synchronization.
+	 */
+	cset = task_css_set(tsk);
+
+	if (!list_empty(&tsk->cg_list)) {
+		spin_lock_irq(&css_set_lock);
+		css_set_move_task(tsk, cset, NULL, false);
+		spin_unlock_irq(&css_set_lock);
+	} else {
+		get_css_set(cset);
+	}
+
+	/* see cgroup_post_fork() for details */
+	do_each_subsys_mask(ss, i, have_exit_callback) {
+		ss->exit(tsk);
+	} while_each_subsys_mask();
+}
+
+void cgroup_free(struct task_struct *task)
+{
+	struct css_set *cset = task_css_set(task);
+	struct cgroup_subsys *ss;
+	int ssid;
+
+	do_each_subsys_mask(ss, ssid, have_free_callback) {
+		ss->free(task);
+	} while_each_subsys_mask();
+
+	put_css_set(cset);
+}
+
+static int __init cgroup_disable(char *str)
+{
+	struct cgroup_subsys *ss;
+	char *token;
+	int i;
+
+	while ((token = strsep(&str, ",")) != NULL) {
+		if (!*token)
+			continue;
+
+		for_each_subsys(ss, i) {
+			if (strcmp(token, ss->name) &&
+			    strcmp(token, ss->legacy_name))
+				continue;
+			cgroup_disable_mask |= 1 << i;
+		}
+	}
+	return 1;
+}
+__setup("cgroup_disable=", cgroup_disable);
+
+/**
+ * css_tryget_online_from_dir - get corresponding css from a cgroup dentry
+ * @dentry: directory dentry of interest
+ * @ss: subsystem of interest
+ *
+ * If @dentry is a directory for a cgroup which has @ss enabled on it, try
+ * to get the corresponding css and return it.  If such css doesn't exist
+ * or can't be pinned, an ERR_PTR value is returned.
+ */
+struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
+						       struct cgroup_subsys *ss)
+{
+	struct kernfs_node *kn = kernfs_node_from_dentry(dentry);
+	struct file_system_type *s_type = dentry->d_sb->s_type;
+	struct cgroup_subsys_state *css = NULL;
+	struct cgroup *cgrp;
+
+	/* is @dentry a cgroup dir? */
+	if ((s_type != &cgroup_fs_type && s_type != &cgroup2_fs_type) ||
+	    !kn || kernfs_type(kn) != KERNFS_DIR)
+		return ERR_PTR(-EBADF);
+
+	rcu_read_lock();
+
+	/*
+	 * This path doesn't originate from kernfs and @kn could already
+	 * have been or be removed at any point.  @kn->priv is RCU
+	 * protected for this access.  See css_release_work_fn() for details.
+	 */
+	cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
+	if (cgrp)
+		css = cgroup_css(cgrp, ss);
+
+	if (!css || !css_tryget_online(css))
+		css = ERR_PTR(-ENOENT);
+
+	rcu_read_unlock();
+	return css;
+}
+
+/**
+ * css_from_id - lookup css by id
+ * @id: the cgroup id
+ * @ss: cgroup subsys to be looked into
+ *
+ * Returns the css if there's valid one with @id, otherwise returns NULL.
+ * Should be called under rcu_read_lock().
+ */
+struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
+{
+	WARN_ON_ONCE(!rcu_read_lock_held());
+	return idr_find(&ss->css_idr, id);
+}
+
+/**
+ * cgroup_get_from_path - lookup and get a cgroup from its default hierarchy path
+ * @path: path on the default hierarchy
+ *
+ * Find the cgroup at @path on the default hierarchy, increment its
+ * reference count and return it.  Returns pointer to the found cgroup on
+ * success, ERR_PTR(-ENOENT) if @path doens't exist and ERR_PTR(-ENOTDIR)
+ * if @path points to a non-directory.
+ */
+struct cgroup *cgroup_get_from_path(const char *path)
+{
+	struct kernfs_node *kn;
+	struct cgroup *cgrp;
+
+	mutex_lock(&cgroup_mutex);
+
+	kn = kernfs_walk_and_get(cgrp_dfl_root.cgrp.kn, path);
+	if (kn) {
+		if (kernfs_type(kn) == KERNFS_DIR) {
+			cgrp = kn->priv;
+			cgroup_get(cgrp);
+		} else {
+			cgrp = ERR_PTR(-ENOTDIR);
+		}
+		kernfs_put(kn);
+	} else {
+		cgrp = ERR_PTR(-ENOENT);
+	}
+
+	mutex_unlock(&cgroup_mutex);
+	return cgrp;
+}
+EXPORT_SYMBOL_GPL(cgroup_get_from_path);
+
+/**
+ * cgroup_get_from_fd - get a cgroup pointer from a fd
+ * @fd: fd obtained by open(cgroup2_dir)
+ *
+ * Find the cgroup from a fd which should be obtained
+ * by opening a cgroup directory.  Returns a pointer to the
+ * cgroup on success. ERR_PTR is returned if the cgroup
+ * cannot be found.
+ */
+struct cgroup *cgroup_get_from_fd(int fd)
+{
+	struct cgroup_subsys_state *css;
+	struct cgroup *cgrp;
+	struct file *f;
+
+	f = fget_raw(fd);
+	if (!f)
+		return ERR_PTR(-EBADF);
+
+	css = css_tryget_online_from_dir(f->f_path.dentry, NULL);
+	fput(f);
+	if (IS_ERR(css))
+		return ERR_CAST(css);
+
+	cgrp = css->cgroup;
+	if (!cgroup_on_dfl(cgrp)) {
+		cgroup_put(cgrp);
+		return ERR_PTR(-EBADF);
+	}
+
+	return cgrp;
+}
+EXPORT_SYMBOL_GPL(cgroup_get_from_fd);
+
+/*
+ * sock->sk_cgrp_data handling.  For more info, see sock_cgroup_data
+ * definition in cgroup-defs.h.
+ */
+#ifdef CONFIG_SOCK_CGROUP_DATA
+
+#if defined(CONFIG_CGROUP_NET_PRIO) || defined(CONFIG_CGROUP_NET_CLASSID)
+
+DEFINE_SPINLOCK(cgroup_sk_update_lock);
+static bool cgroup_sk_alloc_disabled __read_mostly;
+
+void cgroup_sk_alloc_disable(void)
+{
+	if (cgroup_sk_alloc_disabled)
+		return;
+	pr_info("cgroup: disabling cgroup2 socket matching due to net_prio or net_cls activation\n");
+	cgroup_sk_alloc_disabled = true;
+}
+
+#else
+
+#define cgroup_sk_alloc_disabled	false
+
+#endif
+
+void cgroup_sk_alloc(struct sock_cgroup_data *skcd)
+{
+	if (cgroup_sk_alloc_disabled)
+		return;
+
+	/* Socket clone path */
+	if (skcd->val) {
+		cgroup_get(sock_cgroup_ptr(skcd));
+		return;
+	}
+
+	rcu_read_lock();
+
+	while (true) {
+		struct css_set *cset;
+
+		cset = task_css_set(current);
+		if (likely(cgroup_tryget(cset->dfl_cgrp))) {
+			skcd->val = (unsigned long)cset->dfl_cgrp;
+			break;
+		}
+		cpu_relax();
+	}
+
+	rcu_read_unlock();
+}
+
+void cgroup_sk_free(struct sock_cgroup_data *skcd)
+{
+	cgroup_put(sock_cgroup_ptr(skcd));
+}
+
+#endif	/* CONFIG_SOCK_CGROUP_DATA */
+
+#ifdef CONFIG_CGROUP_BPF
+int cgroup_bpf_update(struct cgroup *cgrp, struct bpf_prog *prog,
+		      enum bpf_attach_type type, bool overridable)
+{
+	struct cgroup *parent = cgroup_parent(cgrp);
+	int ret;
+
+	mutex_lock(&cgroup_mutex);
+	ret = __cgroup_bpf_update(cgrp, parent, prog, type, overridable);
+	mutex_unlock(&cgroup_mutex);
+	return ret;
+}
+#endif /* CONFIG_CGROUP_BPF */