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Date: Fri, 4 Mar 2011 22:50:02 +0100
From: Jens Axboe <jaxboe@fusionio.com>
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To: Mike Snitzer <snitzer@redhat.com>
CC: Shaohua Li <shli@kernel.org>,
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Subject: Re: [PATCH 05/10] block: remove per-queue plugging
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On 2011-03-04 22:43, Mike Snitzer wrote:
> On Fri, Mar 04 2011 at  8:02am -0500,
> Shaohua Li <shli@kernel.org> wrote:
> 
>> 2011/3/4 Mike Snitzer <snitzer@redhat.com>:
>>> I'm now hitting a lockdep issue, while running a 'for-2.6.39/stack-plug'
>>> kernel, when I try an fsync heavy workload to a request-based mpath
>>> device (the kernel ultimately goes down in flames, I've yet to look at
>>> the crashdump I took)
>>>
>>>
>>> =======================================================
>>> [ INFO: possible circular locking dependency detected ]
>>> 2.6.38-rc6-snitm+ #2
>>> -------------------------------------------------------
>>> ffsb/3110 is trying to acquire lock:
>>>  (&(&q->__queue_lock)->rlock){..-...}, at: [<ffffffff811b4c4d>] flush_plug_list+0xbc/0x135
>>>
>>> but task is already holding lock:
>>>  (&rq->lock){-.-.-.}, at: [<ffffffff8137132f>] schedule+0x16a/0x725
>>>
>>> which lock already depends on the new lock.
>> I hit this too. Can you check if attached debug patch fixes it?
> 
> Fixes it for me.

The preempt bit in block/ should not be needed. Can you check whether
it's the moving of the flush in sched.c that does the trick?

The problem with the current spot is that it's under the runqueue lock.
The problem with the modified variant is that we flush even if the task
is not going to sleep. We really just want to flush when it is going to
move out of the runqueue, but we want to do that outside of the runqueue
lock as well.

-- 
Jens Axboe

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