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

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Message-ID: <538F3FB5.2020102@kernel.dk>
Date:	Wed, 04 Jun 2014 09:48:05 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Ming Lei <tom.leiming@...il.com>,
	Christoph Hellwig <hch@...radead.org>
CC:	Shaohua Li <shli@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch]blk-mq: blk_mq_tag_to_rq should handle flush request

On 06/04/2014 09:43 AM, Ming Lei wrote:
> On Wed, Jun 4, 2014 at 11:31 PM, Christoph Hellwig <hch@...radead.org> 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;
>> }
> 
> Looks we thought it together, :-)
> 
> Also maybe the flush_rq->tag need to be cleared in flush_end_io().

It clears the command flag, so that should be enough.

-- 
Jens Axboe

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