blkcg: shoot down blkio_groups on elevator switch

Elevator switch may involve changes to blkcg policies.  Implement
shoot down of blkio_groups.

Combined with the previous bypass updates, the end goal is updating
blkcg core such that it can ensure that blkcg's being affected become
quiescent and don't have any per-blkg data hanging around before
commencing any policy updates.  Until queues are made aware of the
policies that applies to them, as an interim step, all per-policy blkg
data will be shot down.

* blk-throtl doesn't need this change as it can't be disabled for a
  live queue; however, update it anyway as the scheduled blkg
  unification requires this behavior change.  This means that
  blk-throtl configuration will be unnecessarily lost over elevator
  switch.  This oddity will be removed after blkcg learns to associate
  individual policies with request_queues.

* blk-throtl dosen't shoot down root_tg.  This is to ease transition.
  Unified blkg will always have persistent root group and not shooting
  down root_tg for now eases transition to that point by avoiding
  having to update td->root_tg and is safe as blk-throtl can never be
  disabled

-v2: Vivek pointed out that group list is not guaranteed to be empty
     on return from clear function if it raced cgroup removal and
     lost.  Fix it by waiting a bit and retrying.  This kludge will
     soon be removed once locking is updated such that blkg is never
     in limbo state between blkcg and request_queue locks.

     blk-throtl no longer shoots down root_tg to avoid breaking
     td->root_tg.

     Also, Nest queue_lock inside blkio_list_lock not the other way
     around to avoid introduce possible deadlock via blkcg lock.

-v3: blkcg_clear_queue() repositioned and renamed to
     blkg_destroy_all() to increase consistency with later changes.
     cfq_clear_queue() updated to check q->elevator before
     dereferencing it to avoid NULL dereference on not fully
     initialized queues (used by later change).

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 33 insertions, 1 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 970a717a056f..159aef59589f 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -17,8 +17,9 @@
 #include <linux/err.h>
 #include <linux/blkdev.h>
 #include <linux/slab.h>
-#include "blk-cgroup.h"
 #include <linux/genhd.h>
+#include <linux/delay.h>
+#include "blk-cgroup.h"
 
 #define MAX_KEY_LEN 100
 
@@ -546,6 +547,37 @@ struct blkio_group *blkiocg_lookup_group(struct blkio_cgroup *blkcg, void *key)
 }
 EXPORT_SYMBOL_GPL(blkiocg_lookup_group);
 
+void blkg_destroy_all(struct request_queue *q)
+{
+	struct blkio_policy_type *pol;
+
+	while (true) {
+		bool done = true;
+
+		spin_lock(&blkio_list_lock);
+		spin_lock_irq(q->queue_lock);
+
+		/*
+		 * clear_queue_fn() might return with non-empty group list
+		 * if it raced cgroup removal and lost.  cgroup removal is
+		 * guaranteed to make forward progress and retrying after a
+		 * while is enough.  This ugliness is scheduled to be
+		 * removed after locking update.
+		 */
+		list_for_each_entry(pol, &blkio_list, list)
+			if (!pol->ops.blkio_clear_queue_fn(q))
+				done = false;
+
+		spin_unlock_irq(q->queue_lock);
+		spin_unlock(&blkio_list_lock);
+
+		if (done)
+			break;
+
+		msleep(10);	/* just some random duration I like */
+	}
+}
+
 static void blkio_reset_stats_cpu(struct blkio_group *blkg)
 {
 	struct blkio_group_stats_cpu *stats_cpu;