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authorTejun Heo <tj@kernel.org>2018-04-26 23:29:05 +0200
committerTejun Heo <tj@kernel.org>2018-04-26 23:29:05 +0200
commit9a9e97b2f1f27ec70e3c138a790bd5627747d868 (patch)
tree3478f3d670cdde6cd9023990c8ebce2e2ecd73a3 /kernel/cgroup/rstat.c
parentcgroup: Add cgroup_subsys->css_rstat_flush() (diff)
downloadlinux-9a9e97b2f1f27ec70e3c138a790bd5627747d868.tar.xz
linux-9a9e97b2f1f27ec70e3c138a790bd5627747d868.zip
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>
Diffstat (limited to 'kernel/cgroup/rstat.c')
-rw-r--r--kernel/cgroup/rstat.c16
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;