memcg: convert mem_cgroup->under_oom from atomic_t to int
memcg->under_oom tracks whether the memcg is under OOM conditions and is
an atomic_t counter managed with mem_cgroup_[un]mark_under_oom(). While
atomic_t appears to be simple synchronization-wise, when used as a
synchronization construct like here, it's trickier and more error-prone
due to weak memory ordering rules, especially around atomic_read(), and
false sense of security.
For example, both non-trivial read sites of memcg->under_oom are a bit
problematic although not being actually broken.
* mem_cgroup_oom_register_event()
It isn't explicit what guarantees the memory ordering between event
addition and memcg->under_oom check. This isn't broken only because
memcg_oom_lock is used for both event list and memcg->oom_lock.
* memcg_oom_recover()
The lockless test doesn't have any explanation why this would be
safe.
mem_cgroup_[un]mark_under_oom() are very cold paths and there's no point
in avoiding locking memcg_oom_lock there. This patch converts
memcg->under_oom from atomic_t to int, puts their modifications under
memcg_oom_lock and documents why the lockless test in
memcg_oom_recover() is safe.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6a5f5d5..e65f7b0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -285,8 +285,9 @@
*/
bool use_hierarchy;
+ /* protected by memcg_oom_lock */
bool oom_lock;
- atomic_t under_oom;
+ int under_oom;
int swappiness;
/* OOM-Killer disable */
@@ -1809,8 +1810,10 @@
{
struct mem_cgroup *iter;
+ spin_lock(&memcg_oom_lock);
for_each_mem_cgroup_tree(iter, memcg)
- atomic_inc(&iter->under_oom);
+ iter->under_oom++;
+ spin_unlock(&memcg_oom_lock);
}
static void mem_cgroup_unmark_under_oom(struct mem_cgroup *memcg)
@@ -1819,11 +1822,13 @@
/*
* When a new child is created while the hierarchy is under oom,
- * mem_cgroup_oom_lock() may not be called. We have to use
- * atomic_add_unless() here.
+ * mem_cgroup_oom_lock() may not be called. Watch for underflow.
*/
+ spin_lock(&memcg_oom_lock);
for_each_mem_cgroup_tree(iter, memcg)
- atomic_add_unless(&iter->under_oom, -1, 0);
+ if (iter->under_oom > 0)
+ iter->under_oom--;
+ spin_unlock(&memcg_oom_lock);
}
static DECLARE_WAIT_QUEUE_HEAD(memcg_oom_waitq);
@@ -1851,7 +1856,15 @@
static void memcg_oom_recover(struct mem_cgroup *memcg)
{
- if (memcg && atomic_read(&memcg->under_oom))
+ /*
+ * For the following lockless ->under_oom test, the only required
+ * guarantee is that it must see the state asserted by an OOM when
+ * this function is called as a result of userland actions
+ * triggered by the notification of the OOM. This is trivially
+ * achieved by invoking mem_cgroup_mark_under_oom() before
+ * triggering notification.
+ */
+ if (memcg && memcg->under_oom)
__wake_up(&memcg_oom_waitq, TASK_NORMAL, 0, memcg);
}
@@ -3860,7 +3873,7 @@
list_add(&event->list, &memcg->oom_notify);
/* already in OOM ? */
- if (atomic_read(&memcg->under_oom))
+ if (memcg->under_oom)
eventfd_signal(eventfd, 1);
spin_unlock(&memcg_oom_lock);
@@ -3889,7 +3902,7 @@
struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(sf));
seq_printf(sf, "oom_kill_disable %d\n", memcg->oom_kill_disable);
- seq_printf(sf, "under_oom %d\n", (bool)atomic_read(&memcg->under_oom));
+ seq_printf(sf, "under_oom %d\n", (bool)memcg->under_oom);
return 0;
}