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

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Date:	Mon, 7 Mar 2011 08:54:15 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Jens Axboe <jaxboe@...ionio.com>
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

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?

Thanks,
Shaohua
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