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Subject[patch 3/3] mm: memcg: fix test for child groups
Date
When memcg code needs to know whether any given memcg has children, it
uses the cgroup child iteration primitives and returns true/false
depending on whether the iteration loop is executed at least once or
not.

Because a cgroup's list of children is RCU protected, these primitives
require the RCU read-lock to be held, which is not the case for all
memcg callers. This results in the following splat when e.g. enabling
hierarchy mode:

[ 3.683974] WARNING: CPU: 3 PID: 1 at /home/hannes/src/linux/linux/kernel/cgroup.c:3043 css_next_child+0xa3/0x160()
[ 3.686266] CPU: 3 PID: 1 Comm: systemd Not tainted 3.12.0-rc5-00117-g83f11a9-dirty #18
[ 3.688616] Hardware name: LENOVO 3680B56/3680B56, BIOS 6QET69WW (1.39 ) 04/26/2012
[ 3.690900] 0000000000000009 ffff88013227bdc8 ffffffff8173602f 0000000000000000
[ 3.693225] ffff88013227be00 ffffffff81090af8 0000000000000000 ffff88013220d000
[ 3.695606] ffff8800b6c50028 ffff88013220d000 0000000000000000 ffff88013227be10
[ 3.697950] Call Trace:
[ 3.700233] [<ffffffff8173602f>] dump_stack+0x54/0x74
[ 3.702503] [<ffffffff81090af8>] warn_slowpath_common+0x78/0xa0
[ 3.704764] [<ffffffff81090c0a>] warn_slowpath_null+0x1a/0x20
[ 3.707009] [<ffffffff81101173>] css_next_child+0xa3/0x160
[ 3.709255] [<ffffffff8118ae7b>] mem_cgroup_hierarchy_write+0x5b/0xa0
[ 3.711497] [<ffffffff810fe428>] cgroup_file_write+0x108/0x2a0
[ 3.713721] [<ffffffff8119b90d>] ? __sb_start_write+0xed/0x1b0
[ 3.715936] [<ffffffff811980fb>] ? vfs_write+0x1bb/0x1e0
[ 3.718155] [<ffffffff810b8d3f>] ? up_write+0x1f/0x40
[ 3.720356] [<ffffffff81197ffd>] vfs_write+0xbd/0x1e0
[ 3.722539] [<ffffffff8119820c>] SyS_write+0x4c/0xa0
[ 3.724685] [<ffffffff817400d2>] system_call_fastpath+0x16/0x1b
[ 3.726809] ---[ end trace ec33c7d4de043d06 ]---

In the memcg case, we only care about children when we are attempting
to modify inheritable attributes interactively. Racing with deletion
could mean a spurious -EBUSY, no problem. Racing with addition is
handled just fine as well through the memcg_create_mutex: if the child
group is not on the list after the mutex is acquired, it won't be
initialized from the parent's attributes until after the unlock.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
mm/memcontrol.c | 35 +++++++++++------------------------
1 file changed, 11 insertions(+), 24 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3e8cd0d9f716..8804be1cb826 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4959,31 +4959,18 @@ static void mem_cgroup_reparent_charges(struct mem_cgroup *memcg)
} while (usage > 0);
}

-/*
- * This mainly exists for tests during the setting of set of use_hierarchy.
- * Since this is the very setting we are changing, the current hierarchy value
- * is meaningless
- */
-static inline bool __memcg_has_children(struct mem_cgroup *memcg)
-{
- struct cgroup_subsys_state *pos;
-
- /* bounce at first found */
- css_for_each_child(pos, &memcg->css)
- return true;
- return false;
-}
-
-/*
- * Must be called with memcg_create_mutex held, unless the cgroup is guaranteed
- * to be already dead (as in mem_cgroup_force_empty, for instance). This is
- * from mem_cgroup_count_children(), in the sense that we don't really care how
- * many children we have; we only need to know if we have any. It also counts
- * any memcg without hierarchy as infertile.
- */
static inline bool memcg_has_children(struct mem_cgroup *memcg)
{
- return memcg->use_hierarchy && __memcg_has_children(memcg);
+ lockdep_assert_held(&memcg_create_mutex);
+ /*
+ * The lock does not prevent addition or deletion to the list
+ * of children, but it prevents a new child from being
+ * initialized based on this parent in css_online(), so it's
+ * enough to decide whether hierarchically inherited
+ * attributes can still be changed or not.
+ */
+ return memcg->use_hierarchy &&
+ !list_empty(&memcg->css.cgroup->children);
}

/*
@@ -5063,7 +5050,7 @@ static int mem_cgroup_hierarchy_write(struct cgroup_subsys_state *css,
*/
if ((!parent_memcg || !parent_memcg->use_hierarchy) &&
(val == 1 || val == 0)) {
- if (!__memcg_has_children(memcg))
+ if (list_empty(&memcg->css.cgroup->children))
memcg->use_hierarchy = val;
else
retval = -EBUSY;
--
1.8.4.2


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