[3.13.y.z extended stable] Patch "mm: memcontrol: do not iterate uninitialized memcgs" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    mm: memcontrol: do not iterate uninitialized memcgs

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.9.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 395b9f01981b904173ed43d3ec0461d25250b50d Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes at cmpxchg.org>
Date: Thu, 2 Oct 2014 16:16:57 -0700
Subject: mm: memcontrol: do not iterate uninitialized memcgs

commit 2f7dd7a4100ad4affcb141605bef178ab98ccb18 upstream.

The cgroup iterators yield css objects that have not yet gone through
css_online(), but they are not complete memcgs at this point and so the
memcg iterators should not return them.  Commit d8ad30559715 ("mm/memcg:
iteration skip memcgs not yet fully initialized") set out to implement
exactly this, but it uses CSS_ONLINE, a cgroup-internal flag that does
not meet the ordering requirements for memcg, and so the iterator may
skip over initialized groups, or return partially initialized memcgs.

The cgroup core can not reasonably provide a clear answer on whether the
object around the css has been fully initialized, as that depends on
controller-specific locking and lifetime rules.  Thus, introduce a
memcg-specific flag that is set after the memcg has been initialized in
css_online(), and read before mem_cgroup_iter() callers access the memcg
members.

Signed-off-by: Johannes Weiner <hannes at cmpxchg.org>
Cc: Tejun Heo <tj at kernel.org>
Acked-by: Michal Hocko <mhocko at suse.cz>
Cc: Hugh Dickins <hughd at google.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
[ kamal: Johannes' 3.12 backport applied to 3.13-stable ]
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 mm/memcontrol.c | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a106a2d..6cbf902 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -251,6 +251,9 @@ struct mem_cgroup {
 	/* vmpressure notifications */
 	struct vmpressure vmpressure;

+	/* css_online() has been completed */
+	int initialized;
+
 	/*
 	 * the counter to account for mem+swap usage.
 	 */
@@ -1108,9 +1111,23 @@ skip_node:
 	 * skipping css reference should be safe.
 	 */
 	if (next_css) {
-		if ((next_css == &root->css) ||
-		    ((next_css->flags & CSS_ONLINE) && css_tryget(next_css)))
-			return mem_cgroup_from_css(next_css);
+		struct mem_cgroup *memcg = mem_cgroup_from_css(next_css);
+
+		if (next_css == &root->css)
+			return memcg;
+
+		if (css_tryget(next_css)) {
+			if (memcg->initialized) {
+				/*
+				 * Make sure the memcg is initialized:
+				 * mem_cgroup_css_online() orders the the
+				 * initialization against setting the flag.
+				 */
+				smp_rmb();
+				return memcg;
+			}
+			css_put(next_css);
+		}

 		prev_css = next_css;
 		goto skip_node;
@@ -6341,6 +6358,16 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)

 	error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
 	mutex_unlock(&memcg_create_mutex);
+
+	if (!error) {
+		/*
+		 * Make sure the memcg is initialized: mem_cgroup_iter()
+		 * orders reading memcg->initialized against its callers
+		 * reading the memcg members.
+		 */
+		smp_wmb();
+		memcg->initialized = 1;
+	}
 	return error;
 }

--
1.9.1