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 14:39:19 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
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()

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.

Cheers,
Jeff
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