livepatch,sched: Add livepatch task switching to cond_resched()

There have been reports [1][2] of live patches failing to complete
within a reasonable amount of time due to CPU-bound kthreads.

Fix it by patching tasks in cond_resched().

There are four different flavors of cond_resched(), depending on the
kernel configuration.  Hook into all of them.

A more elegant solution might be to use a preempt notifier.  However,
non-ORC unwinders can't unwind a preempted task reliably.

[1] https://lore.kernel.org/lkml/20220507174628.2086373-1-song@kernel.org/
[2] https://lkml.kernel.org/lkml/20230120-vhost-klp-switching-v1-0-7c2b65519c43@kernel.org

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
Link: https://lore.kernel.org/r/4ae981466b7814ec221014fc2554b2f86f3fb70b.1677257135.git.jpoimboe@kernel.org

2 files changed, 93 insertions, 15 deletions

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 4bd2d5e10f20..eea7c8ec6e05 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -33,6 +33,7 @@
  *
  * - klp_ftrace_handler()
  * - klp_update_patch_state()
+ * - __klp_sched_try_switch()
  */
 DEFINE_MUTEX(klp_mutex);
 
diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index 824e2e3f07dd..e9fd83a02228 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -9,6 +9,7 @@
 
 #include <linux/cpu.h>
 #include <linux/stacktrace.h>
+#include <linux/static_call.h>
 #include "core.h"
 #include "patch.h"
 #include "transition.h"
@@ -27,6 +28,25 @@ static int klp_target_state = KLP_UNDEFINED;
 static unsigned int klp_signals_cnt;
 
 /*
+ * When a livepatch is in progress, enable klp stack checking in
+ * cond_resched().  This helps CPU-bound kthreads get patched.
+ */
+#if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
+
+#define klp_cond_resched_enable() sched_dynamic_klp_enable()
+#define klp_cond_resched_disable() sched_dynamic_klp_disable()
+
+#else /* !CONFIG_PREEMPT_DYNAMIC || !CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
+
+DEFINE_STATIC_KEY_FALSE(klp_sched_try_switch_key);
+EXPORT_SYMBOL(klp_sched_try_switch_key);
+
+#define klp_cond_resched_enable() static_branch_enable(&klp_sched_try_switch_key)
+#define klp_cond_resched_disable() static_branch_disable(&klp_sched_try_switch_key)
+
+#endif /* CONFIG_PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
+
+/*
  * This work can be performed periodically to finish patching or unpatching any
  * "straggler" tasks which failed to transition in the first attempt.
  */
@@ -174,8 +194,8 @@ void klp_update_patch_state(struct task_struct *task)
 	 * barrier (smp_rmb) for two cases:
 	 *
 	 * 1) Enforce the order of the TIF_PATCH_PENDING read and the
-	 *    klp_target_state read.  The corresponding write barrier is in
-	 *    klp_init_transition().
+	 *    klp_target_state read.  The corresponding write barriers are in
+	 *    klp_init_transition() and klp_reverse_transition().
 	 *
 	 * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
 	 *    of func->transition, if klp_ftrace_handler() is called later on
@@ -343,6 +363,44 @@ static bool klp_try_switch_task(struct task_struct *task)
 	return !ret;
 }
 
+void __klp_sched_try_switch(void)
+{
+	if (likely(!klp_patch_pending(current)))
+		return;
+
+	/*
+	 * This function is called from cond_resched() which is called in many
+	 * places throughout the kernel.  Using the klp_mutex here might
+	 * deadlock.
+	 *
+	 * Instead, disable preemption to prevent racing with other callers of
+	 * klp_try_switch_task().  Thanks to task_call_func() they won't be
+	 * able to switch this task while it's running.
+	 */
+	preempt_disable();
+
+	/*
+	 * Make sure current didn't get patched between the above check and
+	 * preempt_disable().
+	 */
+	if (unlikely(!klp_patch_pending(current)))
+		goto out;
+
+	/*
+	 * Enforce the order of the TIF_PATCH_PENDING read above and the
+	 * klp_target_state read in klp_try_switch_task().  The corresponding
+	 * write barriers are in klp_init_transition() and
+	 * klp_reverse_transition().
+	 */
+	smp_rmb();
+
+	klp_try_switch_task(current);
+
+out:
+	preempt_enable();
+}
+EXPORT_SYMBOL(__klp_sched_try_switch);
+
 /*
  * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
  * Kthreads with TIF_PATCH_PENDING set are woken up.
@@ -449,7 +507,8 @@ void klp_try_complete_transition(void)
 		return;
 	}
 
-	/* we're done, now cleanup the data structures */
+	/* Done!  Now cleanup the data structures. */
+	klp_cond_resched_disable();
 	patch = klp_transition_patch;
 	klp_complete_transition();
 
@@ -501,6 +560,8 @@ void klp_start_transition(void)
 			set_tsk_thread_flag(task, TIF_PATCH_PENDING);
 	}
 
+	klp_cond_resched_enable();
+
 	klp_signals_cnt = 0;
 }
 
@@ -556,8 +617,9 @@ void klp_init_transition(struct klp_patch *patch, int state)
 	 * see a func in transition with a task->patch_state of KLP_UNDEFINED.
 	 *
 	 * Also enforce the order of the klp_target_state write and future
-	 * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
-	 * set a task->patch_state to KLP_UNDEFINED.
+	 * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() and
+	 * __klp_sched_try_switch() don't set a task->patch_state to
+	 * KLP_UNDEFINED.
 	 */
 	smp_wmb();
 
@@ -593,14 +655,10 @@ void klp_reverse_transition(void)
 		 klp_target_state == KLP_PATCHED ? "patching to unpatching" :
 						   "unpatching to patching");
 
-	klp_transition_patch->enabled = !klp_transition_patch->enabled;
-
-	klp_target_state = !klp_target_state;
-
 	/*
 	 * Clear all TIF_PATCH_PENDING flags to prevent races caused by
-	 * klp_update_patch_state() running in parallel with
-	 * klp_start_transition().
+	 * klp_update_patch_state() or __klp_sched_try_switch() running in
+	 * parallel with the reverse transition.
 	 */
 	read_lock(&tasklist_lock);
 	for_each_process_thread(g, task)
@@ -610,9 +668,28 @@ void klp_reverse_transition(void)
 	for_each_possible_cpu(cpu)
 		clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
 
-	/* Let any remaining calls to klp_update_patch_state() complete */
+	/*
+	 * Make sure all existing invocations of klp_update_patch_state() and
+	 * __klp_sched_try_switch() see the cleared TIF_PATCH_PENDING before
+	 * starting the reverse transition.
+	 */
 	klp_synchronize_transition();
 
+	/*
+	 * All patching has stopped, now re-initialize the global variables to
+	 * prepare for the reverse transition.
+	 */
+	klp_transition_patch->enabled = !klp_transition_patch->enabled;
+	klp_target_state = !klp_target_state;
+
+	/*
+	 * Enforce the order of the klp_target_state write and the
+	 * TIF_PATCH_PENDING writes in klp_start_transition() to ensure
+	 * klp_update_patch_state() and __klp_sched_try_switch() don't set
+	 * task->patch_state to the wrong value.
+	 */
+	smp_wmb();
+
 	klp_start_transition();
 }
 
@@ -626,9 +703,9 @@ void klp_copy_process(struct task_struct *child)
 	 * the task flag up to date with the parent here.
 	 *
 	 * The operation is serialized against all klp_*_transition()
-	 * operations by the tasklist_lock. The only exception is
-	 * klp_update_patch_state(current), but we cannot race with
-	 * that because we are current.
+	 * operations by the tasklist_lock. The only exceptions are
+	 * klp_update_patch_state(current) and __klp_sched_try_switch(), but we
+	 * cannot race with them because we are current.
 	 */
 	if (test_tsk_thread_flag(current, TIF_PATCH_PENDING))
 		set_tsk_thread_flag(child, TIF_PATCH_PENDING);