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SubjectRe: [PATCH V2] block: Avoid missed wakeup in request waitqueue
On Tue, May 29, 2012 at 09:15:00AM +0800, Asias He wrote:
> After hot-unplug a stressed disk, I found that rl->wait[] is not empty
> while rl->count[] is empty and there are theads still sleeping on
> get_request after the queue cleanup. With simple debug code, I found
> there are exactly nr_sleep - nr_wakeup of theads in D state. So there
> are missed wakeup.
>
> $ dmesg | grep nr_sleep
> [ 52.917115] ---> nr_sleep=1046, nr_wakeup=873, delta=173
> $ vmstat 1
> 1 173 0 712640 24292 96172 0 0 0 0 419 757 0 0 0 100 0
>
> To quote Tejun:
>
> Ah, okay, freed_request() wakes up single waiter with the assumption
> that after the wakeup there will at least be one successful allocation
> which in turn will continue the wakeup chain until the wait list is
> empty - ie. waiter wakeup is dependent on successful request
> allocation happening after each wakeup. With queue marked dead, any
> woken up waiter fails the allocation path, so the wakeup chaining is
> lost and we're left with hung waiters. What we need is wake_up_all()
> after drain completion.
>
> This patch fixes the missed wakeup by waking up all the theads which
> are sleeping on wait queue after queue drain.
>
> Changes in v2: Drop waitqueue_active() optimization
>
> Signed-off-by: Asias He <asias@redhat.com>

Acked-by: Tejun Heo <tj@kernel.org>

Jens, this one wants Cc: stable.

Thanks.

--
tejun


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