linux-kernel - Re: 2.6.35-rc3 deadlocks on semaphore operations

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Date:	Thu, 24 Jun 2010 21:22:00 +0200
From:	Luca Tettamanti <kronos.it@...il.com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Julia Lawall <julia@...u.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	maciej.rutecki@...il.com
Subject: Re: 2.6.35-rc3 deadlocks on semaphore operations

On Wed, Jun 23, 2010 at 9:14 PM, Luca Tettamanti <kronos.it@...il.com> wrote:
> On Wed, Jun 23, 2010 at 6:29 PM, Manfred Spraul
> <manfred@...orfullife.com> wrote:
>> Hi,
>>
>> I think I found it:
>> Previously, queue.status was never IN_WAKEUP when the semaphore spinlock was
>> held.
>>
>> The last patch changes that:
>> Now the change from IN_WAKEUP to the final result code happens after the the
>> semaphore spinlock is dropped.
>> Thus a task can observe IN_WAKEUP even when it acquired the semaphore
>> spinlock.
>>
>> As a result, semop() sometimes returned 1 (IN_WAKEUP) for a successful
>> operation.
>>
>> Attached is a patch that should fix the bug.
>
> Apache seems fine.

Argh, "seems" was indeed appropriate. Manfred your patch does
alleviate the problem but something is still wrong. I noticed (I'm
developing an ajax heavy web app) that sometimes an apache worker
hangs; I can reproduce the problem with ab (apache benchmark) and a
high concurrency level (I'm testing with 100 and 10k requests, and I
get only 2-5 dropped requests). This does not happen with 2.4.34.
Any idea on how I can debug this further?

Luca
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