On Wed, Sep 11, 2013 at 06:03:57PM +0200, Michal Hocko wrote:
> The patch below. I find it little bit nicer than Hugh's original one
> because having the two checks sounds more confusing.
> What do you think Hugh, Anton?
Acked-by: Anton Vorontsov <anton@xxxxxxxxxx>
Thanks!
> ---
> From 888745909da34f8aee8a208a82d467236b828d0d Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@xxxxxxx>
> Date: Wed, 11 Sep 2013 17:48:10 +0200
> Subject: [PATCH] vmpressure: fix divide-by-0 in vmpressure_work_fn
>
> Hugh Dickins has reported a division by 0 when a vmpressure event is
> processed. The reason for the exception is that a single vmpressure
> work item (which is per memcg) might be processed by multiple CPUs
> because it is enqueued on system_wq which is !WQ_NON_REENTRANT.
> This means that the out of lock vmpr->scanned check in
> vmpressure_work_fn is inherently racy and the racing workers will see
> already zeroed scanned value after they manage to take the spin lock.
>
> The patch simply moves the vmp->scanned check inside the sr_lock to fix
> the race.
>
> The issue was there since the very beginning but "vmpressure: change
> vmpressure::sr_lock to spinlock" might have made it more visible as the
> racing workers would sleep on the mutex and give it more time to see
> updated value. The issue was still there, though.
>
> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> mm/vmpressure.c | 17 +++++++++--------
> 1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> index e0f6283..ad679a0 100644
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -164,18 +164,19 @@ static void vmpressure_work_fn(struct work_struct *work)
> unsigned long scanned;
> unsigned long reclaimed;
>
> + spin_lock(&vmpr->sr_lock);
> +
> /*
> - * Several contexts might be calling vmpressure(), so it is
> - * possible that the work was rescheduled again before the old
> - * work context cleared the counters. In that case we will run
> - * just after the old work returns, but then scanned might be zero
> - * here. No need for any locks here since we don't care if
> - * vmpr->reclaimed is in sync.
> + * Several contexts might be calling vmpressure() and the work
> + * item is sitting on !WQ_NON_REENTRANT workqueue so different
> + * CPUs might execute it concurrently. Bail out if the scanned
> + * counter is already 0 because all the work has been done already.
> */
> - if (!vmpr->scanned)
> + if (!vmpr->scanned) {
> + spin_unlock(&vmpr->sr_lock);
> return;
> + }
>
> - spin_lock(&vmpr->sr_lock);
> scanned = vmpr->scanned;
> reclaimed = vmpr->reclaimed;
> vmpr->scanned = 0;
> --
> 1.7.10.4
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