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Subject[PATCH 8/8] cgroup: Add memory barriers to plug cgroup_rstat_updated() race window
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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>
---
kernel/cgroup/rstat.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index 18b464a..662d7ae 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.
@@ -125,6 +128,13 @@ 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;
--
2.9.5
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