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

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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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