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>

5 files changed, 12 insertions, 74 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index e9e3b038c702..27d39a810cb6 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -500,7 +500,7 @@ static struct blkio_group *blkg_alloc(struct blkio_cgroup *blkcg,
                 return NULL;
 
         spin_lock_init(&blkg->stats_lock);
-        rcu_assign_pointer(blkg->q, q);
+        blkg->q = q;
         INIT_LIST_HEAD(&blkg->q_node);
         blkg->blkcg = blkcg;
         blkg->refcnt = 1;
@@ -611,7 +611,6 @@ struct blkio_group *blkg_lookup_create(struct blkio_cgroup *blkcg,
 
         hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);
         list_add(&blkg->q_node, &q->blkg_list);
-        q->nr_blkgs++;
 
         spin_unlock(&blkcg->lock);
 out:
@@ -648,9 +647,6 @@ static void blkg_destroy(struct blkio_group *blkg)
         list_del_init(&blkg->q_node);
         hlist_del_init_rcu(&blkg->blkcg_node);
 
-        WARN_ON_ONCE(q->nr_blkgs <= 0);
-        q->nr_blkgs--;
-
         /*
          * Put the reference taken at the time of creation so that when all
          * queues are gone, group can be destroyed.
@@ -1232,8 +1228,9 @@ static int blkio_read_blkg_stats(struct blkio_cgroup *blkcg,
         struct hlist_node *n;
         uint64_t cgroup_total = 0;
 
-        rcu_read_lock();
-        hlist_for_each_entry_rcu(blkg, n, &blkcg->blkg_list, blkcg_node) {
+        spin_lock_irq(&blkcg->lock);
+
+        hlist_for_each_entry(blkg, n, &blkcg->blkg_list, blkcg_node) {
                 const char *dname = blkg_dev_name(blkg);
                 int plid = BLKIOFILE_POLICY(cft->private);
 
@@ -1243,15 +1240,16 @@ static int blkio_read_blkg_stats(struct blkio_cgroup *blkcg,
                         cgroup_total += blkio_get_stat_cpu(blkg, plid,
                                                            cb, dname, type);
                 } else {
-                        spin_lock_irq(&blkg->stats_lock);
+                        spin_lock(&blkg->stats_lock);
                         cgroup_total += blkio_get_stat(blkg, plid,
                                                        cb, dname, type);
-                        spin_unlock_irq(&blkg->stats_lock);
+                        spin_unlock(&blkg->stats_lock);
                 }
         }
         if (show_total)
                 cb->fill(cb, "Total", cgroup_total);
-        rcu_read_unlock();
+
+        spin_unlock_irq(&blkcg->lock);
         return 0;
 }
 
@@ -1583,28 +1581,24 @@ static int blkiocg_pre_destroy(struct cgroup_subsys *subsys,
 {
         struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);
 
-        rcu_read_lock();
         spin_lock_irq(&blkcg->lock);
 
         while (!hlist_empty(&blkcg->blkg_list)) {
                 struct blkio_group *blkg = hlist_entry(blkcg->blkg_list.first,
                                                 struct blkio_group, blkcg_node);
-                struct request_queue *q = rcu_dereference(blkg->q);
+                struct request_queue *q = blkg->q;
 
                 if (spin_trylock(q->queue_lock)) {
                         blkg_destroy(blkg);
                         spin_unlock(q->queue_lock);
                 } else {
                         spin_unlock_irq(&blkcg->lock);
-                        rcu_read_unlock();
                         cpu_relax();
-                        rcu_read_lock();
                         spin_lock(&blkcg->lock);
                 }
         }
 
         spin_unlock_irq(&blkcg->lock);
-        rcu_read_unlock();
         return 0;
 }
 
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index df73040a6a5f..66eaefefcbd2 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -176,8 +176,8 @@ struct blkg_policy_data {
 };
 
 struct blkio_group {
-        /* Pointer to the associated request_queue, RCU protected */
-        struct request_queue __rcu *q;
+        /* Pointer to the associated request_queue */
+        struct request_queue *q;
         struct list_head q_node;
         struct hlist_node blkcg_node;
         struct blkio_cgroup *blkcg;
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);
 }
 
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
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index b4d1d4bfc168..33f1b29e53f4 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -365,7 +365,6 @@ struct request_queue {
 #ifdef CONFIG_BLK_CGROUP
         /* XXX: array size hardcoded to avoid include dependency (temporary) */
         struct list_head        blkg_list;
-        int                     nr_blkgs;
 #endif
 
         struct queue_limits     limits;