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 20:36:17 +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 20:33, 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.

-- 
Jens Axboe

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