From 696ac172fffa653dca401bb2b0cad91cf2ce453f Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Thu, 31 Oct 2013 16:34:15 -0700 Subject: [PATCH] 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 Acked-by: Michal Hocko Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 35 +++++++++++------------------------ 1 file changed, 11 insertions(+), 24 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 7e11cb7..e632782 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; -- 2.7.4