path: root/mm
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authorJohannes Weiner <hannes@cmpxchg.org>2013-10-31 16:34:15 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-10-31 16:58:13 -0700
commit696ac172fffa653dca401bb2b0cad91cf2ce453f (patch)
treefec0af0d449705ddfd6145c02cf0420579608d9f /mm
parent0056f4e66a1b8f00245248877e80386af36af14c (diff)
mm: memcg: fix test for child groups
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: WARNING: CPU: 3 PID: 1 at kernel/cgroup.c:3043 css_next_child+0xa3/0x160() CPU: 3 PID: 1 Comm: systemd Not tainted 3.12.0-rc5-00117-g83f11a9-dirty #18 Hardware name: LENOVO 3680B56/3680B56, BIOS 6QET69WW (1.39 ) 04/26/2012 Call Trace: dump_stack+0x54/0x74 warn_slowpath_common+0x78/0xa0 warn_slowpath_null+0x1a/0x20 css_next_child+0xa3/0x160 mem_cgroup_hierarchy_write+0x5b/0xa0 cgroup_file_write+0x108/0x2a0 vfs_write+0xbd/0x1e0 SyS_write+0x4c/0xa0 system_call_fastpath+0x16/0x1b 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> Acked-by: Michal Hocko <mhocko@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm')
1 files changed, 11 insertions, 24 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7e11cb7d75b..e63278222be 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;
retval = -EBUSY;