blkcg: drop unnecessary RCU locking

Now that blkg additions / removals are always done under both q and
blkcg locks, the only places RCU locking is necessary are
blkg_lookup[_create]() for lookup w/o blkcg lock.  This patch drops
unncessary RCU locking replacing it with plain blkcg locking as
necessary.

* blkiocg_pre_destroy() already perform proper locking and don't need
  RCU.  Dropped.

* blkio_read_blkg_stats() now uses blkcg->lock instead of RCU read
  lock.  This isn't a hot path.

* Now unnecessary synchronize_rcu() from queue exit paths removed.
  This makes q->nr_blkgs unnecessary.  Dropped.

* RCU annotation on blkg->q removed.

-v2: Vivek pointed out that blkg_lookup_create() still needs to be
     called under rcu_read_lock().  Updated.

-v3: After the update, stats_lock locking in blkio_read_blkg_stats()
     shouldn't be using _irq variant as it otherwise ends up enabling
     irq while blkcg->lock is locked.  Fixed.

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, 1 insertions, 32 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index e35ee7aeea69..bfa5168249eb 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1046,39 +1046,8 @@ int blk_throtl_init(struct request_queue *q)
 
 void blk_throtl_exit(struct request_queue *q)
 {
-        struct throtl_data *td = q->td;
-        bool wait;
-
-        BUG_ON(!td);
-
+        BUG_ON(!q->td);
         throtl_shutdown_wq(q);
-
-        /* If there are other groups */
-        spin_lock_irq(q->queue_lock);
-        wait = q->nr_blkgs;
-        spin_unlock_irq(q->queue_lock);
-
-        /*
-         * Wait for tg_to_blkg(tg)->q accessors to exit their grace periods.
-         * Do this wait only if there are other undestroyed groups out
-         * there (other than root group). This can happen if cgroup deletion
-         * path claimed the responsibility of cleaning up a group before
-         * queue cleanup code get to the group.
-         *
-         * Do not call synchronize_rcu() unconditionally as there are drivers
-         * which create/delete request queue hundreds of times during scan/boot
-         * and synchronize_rcu() can take significant time and slow down boot.
-         */
-        if (wait)
-                synchronize_rcu();
-
-        /*
-         * Just being safe to make sure after previous flush if some body did
-         * update limits through cgroup and another work got queued, cancel
-         * it.
-         */
-        throtl_shutdown_wq(q);
-
         kfree(q->td);
 }