blkcg: move rcu_read_lock() outside of blkio_group get functions

rcu_read_lock() in throtl_get_tb() and cfq_get_cfqg() holds onto
@blkcg while looking up blkg.  For API cleanup, the next patch will
make the caller responsible for determining @blkcg to look blkg from
and let them specify it as a parameter.  Move rcu read locking out to
the callers to prepare for the change.

-v2: Originally this patch was described as a fix for RCU read locking
     bug around @blkg, which Vivek pointed out to be incorrect.  It
     was from misunderstanding the role of rcu locking as protecting
     @blkg not @blkcg.  Patch description updated.

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, 5 insertions, 6 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 61693d3404d0..6063c4482b86 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1128,13 +1128,10 @@ static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd)
         struct cfq_group *cfqg = NULL, *__cfqg = NULL;
         struct request_queue *q = cfqd->queue;
 
-        rcu_read_lock();
         blkcg = task_blkio_cgroup(current);
         cfqg = cfq_find_cfqg(cfqd, blkcg);
-        if (cfqg) {
-                rcu_read_unlock();
+        if (cfqg)
                 return cfqg;
-        }
 
         /*
          * Need to allocate a group. Allocation of group also needs allocation
@@ -1164,7 +1161,6 @@ static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd)
 
         if (__cfqg) {
                 kfree(cfqg);
-                rcu_read_unlock();
                 return __cfqg;
         }
 
@@ -1172,7 +1168,6 @@ static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd)
                 cfqg = &cfqd->root_group;
 
         cfq_init_add_cfqg_lists(cfqd, cfqg, blkcg);
-        rcu_read_unlock();
         return cfqg;
 }
 
@@ -2870,6 +2865,8 @@ cfq_find_alloc_queue(struct cfq_data *cfqd, bool is_sync,
         struct cfq_group *cfqg;
 
 retry:
+        rcu_read_lock();
+
         cfqg = cfq_get_cfqg(cfqd);
         cic = cfq_cic_lookup(cfqd, ioc);
         /* cic always exists here */
@@ -2885,6 +2882,7 @@ retry:
                         cfqq = new_cfqq;
                         new_cfqq = NULL;
                 } else if (gfp_mask & __GFP_WAIT) {
+                        rcu_read_unlock();
                         spin_unlock_irq(cfqd->queue->queue_lock);
                         new_cfqq = kmem_cache_alloc_node(cfq_pool,
                                         gfp_mask | __GFP_ZERO,
@@ -2910,6 +2908,7 @@ retry:
         if (new_cfqq)
                 kmem_cache_free(cfq_pool, new_cfqq);
 
+        rcu_read_unlock();
         return cfqq;
 }