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Commit 9a9e97b2 authored by Tejun Heo's avatar Tejun Heo
Browse files

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: default avatarTejun Heo <tj@kernel.org>
parent 8f53470b

kernel/cgroup/rstat.c

+13 −3
Original line number Original line Diff line number Diff line
@@ -28,9 +28,12 @@ void cgroup_rstat_updated(struct cgroup *cgrp, int cpu) 
	unsigned long flags;

	unsigned long flags;





	/*

	/*

	 * Speculative already-on-list test.  This may race leading to

	 * Paired with the one in cgroup_rstat_cpu_pop_upated().  Either we

	 * temporary inaccuracies, which is fine.

	 * see NULL updated_next or they see our updated stat.

	 *

	 */

	smp_mb();



	/*

	 * Because @parent's updated_children is terminated with @parent

	 * Because @parent's updated_children is terminated with @parent

	 * instead of NULL, we can tell whether @cgrp is on the list by

	 * instead of NULL, we can tell whether @cgrp is on the list by

	 * testing the next pointer for NULL.

	 * 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;

		*nextp = rstatc->updated_next;

		rstatc->updated_next = NULL;

		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;

	return pos;