linux-kernel - Re: [patch]blk-mq: blk_mq_tag_to

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Date:	Wed, 04 Jun 2014 20:05:33 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Shaohua Li <shli@...nel.org>
CC:	Christoph Hellwig <hch@...radead.org>, linux-kernel@...r.kernel.org
Subject: Re: [patch]blk-mq: blk_mq_tag_to_rq should handle flush request

On 2014-06-04 19:27, Shaohua Li wrote:
> On Wed, Jun 04, 2014 at 10:25:22AM -0600, Jens Axboe wrote:
>> On 06/04/2014 09:47 AM, Jens Axboe wrote:
>>> On 06/04/2014 09:39 AM, Jens Axboe wrote:
>>>> On 06/04/2014 09:31 AM, Christoph Hellwig wrote:
>>>>> On Wed, Jun 04, 2014 at 09:02:19AM -0600, Jens Axboe wrote:
>>>>>>> scsi_mq_find_tag only gets the scsi host, which may have multiple
>>>>>>> queues.  When called from scsi_find_tag we actually have a scsi device,
>>>>>>> so that's not an issue, but when called from scsi_host_find_tag the
>>>>>>> driver only provides the host.
>>>>>>
>>>>>> Only solution I see right now is to have the flush_rq in the shared
>>>>>> tags, but that would potentially be a regression for multiple
>>>>>> devices and heavy flush uses cases. I'll see if I can come up with
>>>>>> something better, or maybe Shaohua has an idea.
>>>>>
>>>>> What about something like the following (untest, uncompiled, maybe
>>>>> pseudo-code):
>>>>>
>>>>> struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
>>>>> {
>>>>> 	struct request *rq = tags->rqs[tag];
>>>>>
>>>>> 	if ((rq->cmd_flags & REQ_FLUSH_SEQ) && rq->q->flush_rq->tag == tag)
>>>>> 		return rq->q->flush_rq;
>>>>> 	return rq;
>>>>
>>>> Ah yes, that'll work, the queue is always assigned. I'll make that change.
>>>
>>> Something like this in complete form. Compile tested only, I'll test it
>>> on dev box. Probably doesn't matter too much, but I prefer to
>>> potentially have the faster path (non-flush) just fall inline.
>>
>> Works for me, committed.
>
> Sounds there is a small race here. FUA request has REQ_FLUSH_SEQ set too.
> Assume its tag is 0. we initialize flush_rq.
> blk_mq_rq_init->blk_rq_init->memset could set flush_rq tag to 0 in a short
> time. In that short time, blk_mq_tag_to_rq will return wrong request for the
> FUA request.
>
> we can do (rq->cmd_flags & REQ_FLUSH_SEQ) && !(rq->cmd_flags & REQ_FUA) in
> is_flush_request to avoid this issue.

We don't memset the entire request anymore from the rq alloc path.

-- 
Jens Axboe

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