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, 9 insertions, 15 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;
 }