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

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Message-ID: <20140605012701.GA13953@kernel.org>
Date:	Thu, 5 Jun 2014 09:27:01 +0800
From:	Shaohua Li <shli@...nel.org>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Christoph Hellwig <hch@...radead.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 10:25:22AM -0600, Jens Axboe wrote:
> 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.

Sounds there is a small race here. FUA request has REQ_FLUSH_SEQ set too.
Assume its tag is 0. we initialize flush_rq.
blk_mq_rq_init->blk_rq_init->memset could set flush_rq tag to 0 in a short
time. In that short time, blk_mq_tag_to_rq will return wrong request for the
FUA request.

we can do (rq->cmd_flags & REQ_FLUSH_SEQ) && !(rq->cmd_flags & REQ_FUA) in
is_flush_request to avoid this issue.

Thanks,
Shaohua
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