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

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Message-ID: <CACVXFVM-aJwBgrvgT-nNbbi81vE4qeS1LgsS5cFGiz5cbsApZw@mail.gmail.com>
Date:	Thu, 5 Jun 2014 00:00:22 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Christoph Hellwig <hch@...radead.org>,
	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 Wed, Jun 4, 2014 at 11:48 PM, Jens Axboe <axboe@...nel.dk> wrote:
> 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.

Only the flush_rq's command flag is cleared, and its parent request
flag isn't cleared.


Thanks,
-- 
Ming Lei
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