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

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Message-ID: <20140604153159.GA27096@infradead.org>
Date:	Wed, 4 Jun 2014 08:31:59 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Christoph Hellwig <hch@...radead.org>,
	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 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;
}

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