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SubjectRe: [PATCH V2] block: Avoid missed wakeup in request waitqueue
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@redhat.com>
>
> Acked-by: Tejun Heo<tj@kernel.org>
>
> Jens, this one wants Cc: stable.

Ping.

--
Asias


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