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, 0 insertions, 24 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 393eaa59913b..9e386d9bcb79 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3449,7 +3449,6 @@ static void cfq_exit_queue(struct elevator_queue *e)
 {
         struct cfq_data *cfqd = e->elevator_data;
         struct request_queue *q = cfqd->queue;
-        bool wait = false;
 
         cfq_shutdown_timer_wq(cfqd);
 
@@ -3462,31 +3461,8 @@ static void cfq_exit_queue(struct elevator_queue *e)
 
         spin_unlock_irq(q->queue_lock);
 
-#ifdef CONFIG_BLK_CGROUP
-        /*
-         * If there are groups which we could not unlink from blkcg list,
-         * wait for a rcu period for them to be freed.
-         */
-        spin_lock_irq(q->queue_lock);
-        wait = q->nr_blkgs;
-        spin_unlock_irq(q->queue_lock);
-#endif
         cfq_shutdown_timer_wq(cfqd);
 
-        /*
-         * Wait for cfqg->blkg->key accessors to exit their grace periods.
-         * Do this wait only if there are other unlinked groups out
-         * there. 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();
-
 #ifndef CONFIG_CFQ_GROUP_IOSCHED
         kfree(cfqd->root_group);
 #endif