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

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Date:	Wed, 04 Jun 2014 10:25:22 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Shaohua Li <shli@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [patch]blk-mq: blk_mq_tag_to_rq should handle flush request

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.

-- 
Jens Axboe

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