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Subject[PATCH 2/2] memcg: barriers to see memcgs as fully initialized
Commit d8ad30559715 ("mm/memcg: iteration skip memcgs not yet fully
initialized") is not bad, but Greg Thelen asks "Are barriers needed?"

Yes, I'm afraid so: this makes it a little heavier than the original,
but there's no point in guaranteeing that mem_cgroup_iter() returns only
fully initialized memcgs, if we don't guarantee that the initialization
is visible.

If we move online_css()'s setting CSS_ONLINE after rcu_assign_pointer()
(I don't see why not), we can reasonably rely on the smp_wmb() in that.
But I can't find a pre-existing barrier at the mem_cgroup_iter() end,
so add an smp_rmb() where __mem_cgroup_iter_next() returns non-NULL.

Fixes: d8ad30559715 ("mm/memcg: iteration skip memcgs not yet fully initialized")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org # 3.12+
---
I'd have been happier not to have to add this patch: maybe you can see
a better placement, or a way we can avoid this altogether.

kernel/cgroup.c | 8 +++++++-
mm/memcontrol.c | 11 +++++++++--
2 files changed, 16 insertions(+), 3 deletions(-)

--- 3.14-rc2+/kernel/cgroup.c 2014-02-02 18:49:07.737302111 -0800
+++ linux/kernel/cgroup.c 2014-02-12 11:59:52.804041895 -0800
@@ -4063,9 +4063,15 @@ static int online_css(struct cgroup_subs
if (ss->css_online)
ret = ss->css_online(css);
if (!ret) {
- css->flags |= CSS_ONLINE;
css->cgroup->nr_css++;
rcu_assign_pointer(css->cgroup->subsys[ss->subsys_id], css);
+ /*
+ * Set CSS_ONLINE after rcu_assign_pointer(), so that its
+ * smp_wmb() will guarantee that those seeing CSS_ONLINE
+ * can see the initialization done in ss->css_online() - if
+ * they provide an smp_rmb(), as in __mem_cgroup_iter_next().
+ */
+ css->flags |= CSS_ONLINE;
}
return ret;
}
--- 3.14-rc2+/mm/memcontrol.c 2014-02-12 11:55:02.836035004 -0800
+++ linux/mm/memcontrol.c 2014-02-12 11:59:52.804041895 -0800
@@ -1128,9 +1128,16 @@ skip_node:
*/
if (next_css) {
if ((next_css == &root->css) ||
- ((next_css->flags & CSS_ONLINE) && css_tryget(next_css)))
+ ((next_css->flags & CSS_ONLINE) && css_tryget(next_css))) {
+ /*
+ * Ensure that all memcg initialization, done before
+ * CSS_ONLINE was set, will be visible to our caller.
+ * This matches the smp_wmb() in online_css()'s
+ * rcu_assign_pointer(), before it set CSS_ONLINE.
+ */
+ smp_rmb();
return mem_cgroup_from_css(next_css);
-
+ }
prev_css = next_css;
goto skip_node;
}

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