Re: [PATCH V2] block: Avoid missed wakeup in request waitqueue

[Asias He]

Hello, Jens

On 05/29/2012 09:21 AM, Tejun Heo wrote:
> 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@xxxxxxxxxx>
>
> Acked-by: Tejun Heo<tj@xxxxxxxxxx>
>
> Jens, this one wants Cc: stable.

Ping.

--
Asias
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