1 files changed, 21 insertions, 19 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 5e2723f2c6a3..2ff74ffcbb27 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -29,6 +29,14 @@
 
 #define MAX_KEY_LEN 100
 
+/*
+ * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
+ * blkcg_pol_register_mutex nests outside of it and synchronizes entire
+ * policy [un]register operations including cgroup file additions /
+ * removals.  Putting cgroup file registration outside blkcg_pol_mutex
+ * allows grabbing it from cgroup callbacks.
+ */
+static DEFINE_MUTEX(blkcg_pol_register_mutex);
 static DEFINE_MUTEX(blkcg_pol_mutex);
 
 struct blkcg blkcg_root;
@@ -453,20 +461,7 @@ static int blkcg_reset_stats(struct cgroup_subsys_state *css,
         struct blkcg_gq *blkg;
         int i;
 
-        /*
-         * 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();
+        mutex_lock(&blkcg_pol_mutex);
         spin_lock_irq(&blkcg->lock);
 
         /*
@@ -1190,6 +1185,7 @@ int blkcg_policy_register(struct blkcg_policy *pol)
         if (WARN_ON(pol->pd_size < sizeof(struct blkg_policy_data)))
                 return -EINVAL;
 
+        mutex_lock(&blkcg_pol_register_mutex);
         mutex_lock(&blkcg_pol_mutex);
 
         /* find an empty slot */
@@ -1198,19 +1194,23 @@ int blkcg_policy_register(struct blkcg_policy *pol)
                 if (!blkcg_policy[i])
                         break;
         if (i >= BLKCG_MAX_POLS)
-                goto out_unlock;
+                goto err_unlock;
 
         /* register and update blkgs */
         pol->plid = i;
         blkcg_policy[i] = pol;
+        mutex_unlock(&blkcg_pol_mutex);
 
         /* everything is in place, add intf files for the new policy */
         if (pol->cftypes)
                 WARN_ON(cgroup_add_legacy_cftypes(&blkio_cgrp_subsys,
                                                   pol->cftypes));
-        ret = 0;
-out_unlock:
+        mutex_unlock(&blkcg_pol_register_mutex);
+        return 0;
+
+err_unlock:
         mutex_unlock(&blkcg_pol_mutex);
+        mutex_unlock(&blkcg_pol_register_mutex);
         return ret;
 }
 EXPORT_SYMBOL_GPL(blkcg_policy_register);
@@ -1223,7 +1223,7 @@ EXPORT_SYMBOL_GPL(blkcg_policy_register);
  */
 void blkcg_policy_unregister(struct blkcg_policy *pol)
 {
-        mutex_lock(&blkcg_pol_mutex);
+        mutex_lock(&blkcg_pol_register_mutex);
 
         if (WARN_ON(blkcg_policy[pol->plid] != pol))
                 goto out_unlock;
@@ -1233,8 +1233,10 @@ void blkcg_policy_unregister(struct blkcg_policy *pol)
                 cgroup_rm_cftypes(pol->cftypes);
 
         /* unregister and update blkgs */
+        mutex_lock(&blkcg_pol_mutex);
         blkcg_policy[pol->plid] = NULL;
-out_unlock:
         mutex_unlock(&blkcg_pol_mutex);
+out_unlock:
+        mutex_unlock(&blkcg_pol_register_mutex);
 }
 EXPORT_SYMBOL_GPL(blkcg_policy_unregister);