On Wed 03-06-15 11:38:59, Tejun Heo wrote:
> From 5456f353297d6f10b45fd794674b09dd5ab502ca Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@xxxxxxxxxx>
> Date: Tue, 2 Jun 2015 09:29:11 -0400
>
> 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@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxx>
> ---
> mm/memcontrol.c | 29 +++++++++++++++++++++--------
> 1 file changed, 21 insertions(+), 8 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9f39647..4de6647 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -285,8 +285,9 @@ struct mem_cgroup {
> */
> 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 @@ static void mem_cgroup_mark_under_oom(struct mem_cgroup *memcg)
> {
> 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 @@ static void mem_cgroup_unmark_under_oom(struct mem_cgroup *memcg)
>
> /*
> * 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);
> @@ -1857,7 +1862,15 @@ static void memcg_wakeup_oom(struct mem_cgroup *memcg)
>
> 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)
> memcg_wakeup_oom(memcg);
> }
>
> @@ -3866,7 +3879,7 @@ static int mem_cgroup_oom_register_event(struct mem_cgroup *memcg,
> 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);
>
> @@ -3895,7 +3908,7 @@ static int mem_cgroup_oom_control_read(struct seq_file *sf, void *v)
> 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;
> }
>
> --
> 2.4.2
>
--
Michal Hocko
SUSE Labs
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