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Re: [2.6.36-rc3] Workqueues, XFS, dependencies and deadlocks

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [2.6.36-rc3] Workqueues, XFS, dependencies and deadlocks
From: Tejun Heo <tj@xxxxxxxxxx>
Date: Tue, 07 Sep 2010 14:26:54 +0200
Cc: linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx
In-reply-to: <4C861582.6080102@xxxxxxxxxx>
References: <20100907072954.GM705@dastard> <4C86003B.6090706@xxxxxxxxxx> <20100907100108.GN705@dastard> <4C861582.6080102@xxxxxxxxxx>
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv:1.9.2.8) Gecko/20100802 Thunderbird/3.1.2
On 09/07/2010 12:35 PM, Tejun Heo wrote:
> Can you please help me a bit more?  Are you saying the following?
> 
> Work w0 starts execution on wq0.  w0 tries locking but fails.  Does
> delay(1) and requeues itself on wq0 hoping another work w1 would be
> queued on wq0 which will release the lock.  The requeueing should make
> w0 queued and executed after w1, but instead w1 never gets executed
> while w0 hogs the CPU constantly by re-executing itself.  Also, how
> does delay(1) help with chewing up CPU?  Are you talking about
> avoiding constant lock/unlock ops starving other lockers?  In such
> case, wouldn't cpu_relax() make more sense?

Ooh, almost forgot.  There was nr_active underflow bug in workqueue
code which could lead to malfunctioning max_active regulation and
problems during queue freezing, so you could be hitting that too.  I
sent out pull request some time ago but hasn't been pulled into
mainline yet.  Can you please pull from the following branch and add
WQ_HIGHPRI as discussed before and see whether the problem is still
reproducible?  And if the problem is reproducible, can you please
trigger sysrq thread dump and attach it?

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git for-linus

Thanks.

-- 
tejun

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