Re: [PATCH] execute the whole memcg freeing in rcu callback

[Greg Thelen <gthelen@google.com>]

On Mon, Aug 13, 2012 at 5:46 AM, Glauber Costa <glommer@parallels.com> wrote:
> A lot of the initialization we do in mem_cgroup_create() is done with
> softirqs enabled. This include grabbing a css id, which holds
> &ss->id_lock->rlock, and the per-zone trees, which holds
> rtpz->lock->rlock. All of those signal to the lockdep mechanism that
> those locks can be used in SOFTIRQ-ON-W context. This means that the
> freeing of memcg structure must happen in a compatible context,
> otherwise we'll get a deadlock.
>
> The reference counting mechanism we use allows the memcg structure to be
> freed later and outlive the actual memcg destruction from the
> filesystem. However, we have little, if any, means to guarantee in which
> context the last memcg_put will happen. The best we can do is test it
> and try to make sure no invalid context releases are happening. But as
> we add more code to memcg, the possible interactions grow in number and
> expose more ways to get context conflicts.
>
> Context-related problems already appeared for static branches
> destruction, since their locking forced us to disable them from process
> context, which we could not always guarantee. Now that we're trying to
> add kmem controller, the possibilities of where the freeing can be
> triggered from just increases.
>
> Greg Thelen reported a bug with that patchset applied that would trigger
> if a task would hold a reference to a memcg through its kmem counter.
> This would mean that killing that task would eventually get us to
> __mem_cgroup_free() after dropping the last kernel page reference, in an
> invalid IN-SOFTIRQ-W.
>
> We already moved a part of the freeing to a worker thread to be
> context-safe for the static branches disabling. Although we could move
> the new offending part to such a place as well, I see no reason not
> to do it for the whole freeing action. I consider this to be the safe
> choice.
>
> Signed-off-by: Glauber Costa <glommer@parallels.com>
> Reported-by: Greg Thelen <gthelen@google.com>
> CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> CC: Michal Hocko <mhocko@suse.cz>
> CC: Johannes Weiner <hannes@cmpxchg.org>

The problem I reported is fixed by this patch.  Thanks.

Tested-by: Greg Thelen <gthelen@google.com>

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