linux-kernel - Re: [PATCH v2.6.38-rc5 2/2] block: blk-flush shouldn't call directly into q->request_fn() __blk_run

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Date:	Mon, 07 Mar 2011 20:41:10 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	Tejun Heo <tj@...nel.org>, Mike Snitzer <snitzer@...il.com>,
	Jan Beulich <JBeulich@...ell.com>,
	"David S. Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org
Subject: Re: [PATCH v2.6.38-rc5 2/2] block: blk-flush shouldn't call directly
 into q->request_fn() __blk_run_queue()

On 2011-03-07 20:39, Jeff Moyer wrote:
> Jens Axboe <axboe@...nel.dk> writes:
> 
>> On 2011-03-07 20:33, Jeff Moyer wrote:
>>> Tejun Heo <tj@...nel.org> writes:
>>>
>>>> Hello, Jens.
>>>>
>>>> On Wed, Mar 02, 2011 at 08:46:46AM -0500, Jens Axboe wrote:
>>>>>> Right, thanks.  Jens, after you apply the two fixes for 2.6.38, I can
>>>>>> create a merge branch for for-2.6.39/core which you can pull.  Would
>>>>>> that work for you?
>>>>>
>>>>> Thanks, that would be great. I'm applying them now.
>>>>
>>>> Okay, please pull from the following branch to receive the merge
>>>> between linux-2.6-block:for-linus and :for-2.6.39/core.
>>>>
>>>>  git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git block-for-2.6.39-core
>>>>
>>>> HEAD is e83a46bbb1d4c03defd733a64b727632a40059ad but git.korg seems a
>>>> bit slow to sync, so if you don't see the commit there, please pull
>>>> from master.korg.
>>>>
>>>>  ssh://master.kernel.org/pub/scm/linux/kernel/git/tj/misc.git block-for-2.6.39-core
>>>>
>>>> Thanks.
>>>
>>> I know I'm coming to the party late (and maybe wrong), but I've got some
>>> questions here.
>>>
>>> Tejun, you introduced a commit to the ide driver that made it block in
>>> its request function.  As far as I know, that's not allowed.  For scsi,
>>> at least, it has always allowed calling back into the request function
>>> from the completion handler, and I think this is actully the common case
>>> (not some corner case).
>>>
>>> So, why doesn't the ide driver see calls back into its request function
>>> from the completion handler?  It's clear that it calls blk_end_request
>>> from ide_end_rq, which can definitely call __blk_run_queue.  In other
>>> words, why is it that the flush requests are triggerring this problem
>>> while normal I/O isn't?
>>>
>>> I think the real issue may just be that the ide driver is blocking in
>>> its request function.  What have I missed?
>>
>> So the only case where the request_fn is called and you cannot block, is
>> if you call it from your completion function. Any other invocation
>> should be from process context. As long as you remember to drop the
>> queue lock and re-enable interrupts, it should work. It's not great
>> style and I would not recommend it for a performance environment, but it
>> should work.
> 
> So are you agreeing with me or disagreeing?  ;-)  It sounds to me like
> you're saying that the ide driver should be able to cope with being
> called from softirq context.

I'm just stating how it should work :-)

But yes, it sounds like IDE is violating this rule and that's why it was
broken. Even with that, having explicit control of the queue running
does make sense.

-- 
Jens Axboe

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