linux-kernel - Re: [PATCH 05/10] block: remove per-queue plugging

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Message-ID: <4D749247.3020105@fusionio.com>
Date:	Mon, 7 Mar 2011 09:07:35 +0100
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Shaohua Li <shli@...nel.org>
CC:	Mike Snitzer <snitzer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>
Subject: Re: [PATCH 05/10] block: remove per-queue plugging

On 2011-03-07 01:54, Shaohua Li wrote:
> 2011/3/5 Jens Axboe <jaxboe@...ionio.com>:
>> On 2011-03-04 22:43, Mike Snitzer wrote:
>>> On Fri, Mar 04 2011 at  8:02am -0500,
>>> Shaohua Li <shli@...nel.org> wrote:
>>>
>>>> 2011/3/4 Mike Snitzer <snitzer@...hat.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?
> yes, it's not related to the lockdep issue. but I think we still need
> it. if there is a preempt  between attempt_plub_merge(), we do queue
> flush, then we might hit an incomplete list of request->biotail. Am I
> missing anything?

Ah, so it is needed with the other fix you proposed, since we do flush
on preempt then. If we only do the flush on going to sleep, then we
don't need that preemption disable in that section.

-- 
Jens Axboe

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