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

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Message-ID: <538F3F82.5060805@kernel.dk>
Date:	Wed, 04 Jun 2014 09:47:14 -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 09:39 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.


-- 
Jens Axboe


View attachment "tag-to-request.patch" of type "text/x-patch" (2580 bytes)