linux-kernel - Re: [PATCH] blk-mq: fix blk_mq_tagset_busy

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Date:   Thu, 02 Aug 2018 10:18:38 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Jens Axboe <axboe@...nel.dk>, Ming Lei <ming.lei@...hat.com>
Cc:     linux-block@...r.kernel.org, Josef Bacik <josef@...icpanda.com>,
        Christoph Hellwig <hch@....de>,
        Guenter Roeck <linux@...ck-us.net>,
        Mark Brown <broonie@...nel.org>,
        Matt Hart <matthew.hart@...aro.org>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        John Garry <john.garry@...wei.com>,
        Hannes Reinecke <hare@...e.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-mq: fix blk_mq_tagset_busy_iter

On Thu, 2018-08-02 at 11:08 -0600, Jens Axboe wrote:
> On 8/2/18 11:06 AM, Ming Lei wrote:
> > On Thu, Aug 02, 2018 at 09:54:06AM -0700, James Bottomley wrote:
> > > On Fri, 2018-08-03 at 00:43 +0800, Ming Lei wrote:
> > > > Commit d250bf4e776ff09d5("blk-mq: only iterate over inflight
> > > > requests
> > > > in blk_mq_tagset_busy_iter") uses 'blk_mq_rq_state(rq) ==
> > > > MQ_RQ_IN_FLIGHT' to replace 'blk_mq_request_started(req)', this
> > > > way is wrong, and causes lots of test system hang during
> > > > booting.
> > > > 
> > > > Fix the issue by using blk_mq_request_started(req) inside
> > > > bt_tags_iter().
> > > > 
> > > > Fixes: d250bf4e776ff09d5 ("blk-mq: only iterate over inflight
> > > > requests in blk_mq_tagset_busy_iter")
> > > > Cc: Josef Bacik <josef@...icpanda.com>
> > > > Cc: Christoph Hellwig <hch@....de>
> > > > Cc: Guenter Roeck <linux@...ck-us.net>
> > > > Cc: Mark Brown <broonie@...nel.org>
> > > > Cc: Matt Hart <matthew.hart@...aro.org>
> > > > Cc: Johannes Thumshirn <jthumshirn@...e.de>
> > > > Cc: John Garry <john.garry@...wei.com>
> > > > Cc: Hannes Reinecke <hare@...e.com>,
> > > > Cc: "Martin K. Petersen" <martin.petersen@...cle.com>,
> > > > Cc: James Bottomley <James.Bottomley@...senpartnership.com>
> > > > Cc: linux-scsi@...r.kernel.org
> > > > Cc: linux-kernel@...r.kernel.org
> > > > Signed-off-by: Ming Lei <ming.lei@...hat.com>
> > > > ---
> > > >  block/blk-mq-tag.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> > > > index 09b2ee6694fb..3de0836163c2 100644
> > > > --- a/block/blk-mq-tag.c
> > > > +++ b/block/blk-mq-tag.c
> > > > @@ -271,7 +271,7 @@ static bool bt_tags_iter(struct sbitmap
> > > > *bitmap,
> > > > unsigned int bitnr, void *data)
> > > >  	 * test and set the bit before assining ->rqs[].
> > > >  	 */
> > > >  	rq = tags->rqs[bitnr];
> > > > -	if (rq && blk_mq_rq_state(rq) == MQ_RQ_IN_FLIGHT)
> > > > +	if (rq && blk_mq_request_started(rq))
> > > 
> > > So now we have dueling versions of this patch:
> > > 
> > > https://marc.info/?l=linux-scsi&m=153322802207688
> > > 
> > > Can we at least make sure we've root caused the problem and
> > > confirmed we've got it fixed before we start the formal patch
> > > process?  When we
> > 
> > EH uses scsi_host_busy to check if the error handler needs to be
> > waken up. And blk_mq_tagset_busy_iter() is used for implementing
> > scsi_host_busy(), so causes EH not waken up, then this timed-out
> > request can't be handled.

Yes, I know what the problem is and why this patch is necessary and
that it is very likely the root cause.  However, can we confirm that it
fixes the boot hang completely before we declare victory?

> > > do start the formal patch process, please give appropriate credit
> > > to the reporter(s) since this has been a royal pain for them to
> > > help us track down.
> > 
> > Sure.
> > 
> > Jens, could you add reported-by if you are fine with this version?
> > Or please just let me know if new version is needed, then I can add
> > it.
> 
> I'll add that, would also love a tested-by from the reporter. The
> patch looks good to me, however.

Is there a reason why blk_mq_request_started() isn't a static inline? 
It looks to be somewhat in the hot path.

James