cgroup: Add memory barriers to plug cgroup_rstat_updated() race window

cgroup_rstat_updated() has a small race window where an updated
signaling can race with flush and could be lost till the next update.
This wasn't a problem for the existing usages, but we plan to use
rstat to track counters which need to be accurate.

This patch plugs the race window by synchronizing
cgroup_rstat_updated() and flush path with memory barriers around
cgroup_rstat_cpu->updated_next pointer.

Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 13 insertions, 3 deletions

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 339366e257d4..a4901e20735a 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -28,9 +28,12 @@ void cgroup_rstat_updated(struct cgroup *cgrp, int cpu)
         unsigned long flags;
 
         /*
-         * Speculative already-on-list test.  This may race leading to
-         * temporary inaccuracies, which is fine.
-         *
+         * Paired with the one in cgroup_rstat_cpu_pop_upated().  Either we
+         * see NULL updated_next or they see our updated stat.
+         */
+        smp_mb();
+
+        /*
          * Because @parent's updated_children is terminated with @parent
          * instead of NULL, we can tell whether @cgrp is on the list by
          * testing the next pointer for NULL.
@@ -126,6 +129,13 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
 
                 *nextp = rstatc->updated_next;
                 rstatc->updated_next = NULL;
+
+                /*
+                 * Paired with the one in cgroup_rstat_cpu_updated().
+                 * Either they see NULL updated_next or we see their
+                 * updated stat.
+                 */
+                smp_mb();
         }
 
         return pos;