1 files changed, 14 insertions, 1 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index e4a4145926f6..1039fb9ff5f5 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -451,7 +451,20 @@ static int blkcg_reset_stats(struct cgroup_subsys_state *css,
         struct blkcg_gq *blkg;
         int i;
 
-        mutex_lock(&blkcg_pol_mutex);
+        /*
+         * XXX: We invoke cgroup_add/rm_cftypes() under blkcg_pol_mutex
+         * which ends up putting cgroup's internal cgroup_tree_mutex under
+         * it; however, cgroup_tree_mutex is nested above cgroup file
+         * active protection and grabbing blkcg_pol_mutex from a cgroup
+         * file operation creates a possible circular dependency.  cgroup
+         * internal locking is planned to go through further simplification
+         * and this issue should go away soon.  For now, let's trylock
+         * blkcg_pol_mutex and restart the write on failure.
+         *
+         * http://lkml.kernel.org/g/5363C04B.4010400@oracle.com
+         */
+        if (!mutex_trylock(&blkcg_pol_mutex))
+                return restart_syscall();
         spin_lock_irq(&blkcg->lock);
 
         /*