blkcg: blkcg_css_alloc() should grab blkcg_pol_mutex while iterating blkcg_policy[]

An entry in blkcg_policy[] is stable while there are non-bypassing
in-flight IOs on a request_queue which has the policy activated.  This
is why most derefs of blkcg_policy[] don't need explicit locking;
however, blkcg_css_alloc() isn't invoked from IO path and thus doesn't
have this protection and may race policies being added and removed.

Fix it by adding explicit blkcg_pol_mutex protection around
blkcg_policy[] iteration in blkcg_css_alloc().

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: e48453c386f3 ("block, cgroup: implement policy-specific per-blkcg data")
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Arianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-cgroup.c

@@ -844,6 +844,8 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
 		goto free_blkcg;
 	}
 
+	mutex_lock(&blkcg_pol_mutex);
+
 	for (i = 0; i < BLKCG_MAX_POLS ; i++) {
 		struct blkcg_policy *pol = blkcg_policy[i];
 		struct blkcg_policy_data *cpd;
@@ -860,6 +862,7 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
 		BUG_ON(blkcg->pd[i]);
 		cpd = kzalloc(pol->cpd_size, GFP_KERNEL);
 		if (!cpd) {
+			mutex_unlock(&blkcg_pol_mutex);
 			ret = ERR_PTR(-ENOMEM);
 			goto free_pd_blkcg;
 		}
@@ -868,6 +871,7 @@ blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
 		pol->cpd_init_fn(blkcg);
 	}
 
+	mutex_unlock(&blkcg_pol_mutex);
 done:
 	spin_lock_init(&blkcg->lock);
 	INIT_RADIX_TREE(&blkcg->blkg_tree, GFP_ATOMIC);