RE: [PATCH 1/1] [SCSI] Fix a bug in deriving the FLUSH_TIMEOUT from the basic I/O timeout

[KY Srinivasan <kys@microsoft.com>]

> -----Original Message-----
> From: Mike Christie [mailto:michaelc@cs.wisc.edu]
> Sent: Thursday, June 5, 2014 6:33 PM
> To: KY Srinivasan
> Cc: James Bottomley; linux-kernel@vger.kernel.org; apw@canonical.com;
> devel@linuxdriverproject.org; hch@infradead.org; linux-
> scsi@vger.kernel.org; ohering@suse.com; gregkh@linuxfoundation.org;
> jasowang@redhat.com
> Subject: Re: [PATCH 1/1] [SCSI] Fix a bug in deriving the FLUSH_TIMEOUT
> from the basic I/O timeout
> 
> On 06/04/2014 12:15 PM, KY Srinivasan wrote:
> >
> >
> >> -----Original Message-----
> >> From: James Bottomley [mailto:jbottomley@parallels.com]
> >> Sent: Wednesday, June 4, 2014 10:02 AM
> >> To: KY Srinivasan
> >> Cc: linux-kernel@vger.kernel.org; apw@canonical.com;
> >> devel@linuxdriverproject.org; hch@infradead.org; linux-
> >> scsi@vger.kernel.org; ohering@suse.com; gregkh@linuxfoundation.org;
> >> jasowang@redhat.com
> >> Subject: Re: [PATCH 1/1] [SCSI] Fix a bug in deriving the
> >> FLUSH_TIMEOUT from the basic I/O timeout
> >>
> >> On Wed, 2014-06-04 at 09:33 -0700, K. Y. Srinivasan wrote:
> >>> Commit ID: 7e660100d85af860e7ad763202fff717adcdaacd added code to
> >>> derive the FLUSH_TIMEOUT from the basic I/O timeout. However, this
> >>> patch did not use the basic I/O timeout of the device. Fix this bug.
> >>>
> >>> Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
> >>> ---
> >>>  drivers/scsi/sd.c |    4 +++-
> >>>  1 files changed, 3 insertions(+), 1 deletions(-)
> >>>
> >>> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c index
> >>> e9689d5..54150b1 100644
> >>> --- a/drivers/scsi/sd.c
> >>> +++ b/drivers/scsi/sd.c
> >>> @@ -832,7 +832,9 @@ static int sd_setup_write_same_cmnd(struct
> >>> scsi_device *sdp, struct request *rq)
> >>>
> >>>  static int scsi_setup_flush_cmnd(struct scsi_device *sdp, struct
> >>> request *rq)  {
> >>> -	rq->timeout *= SD_FLUSH_TIMEOUT_MULTIPLIER;
> >>> +	int timeout = sdp->request_queue->rq_timeout;
> >>> +
> >>> +	rq->timeout = (timeout * SD_FLUSH_TIMEOUT_MULTIPLIER);
> >>
> >> Could you share where you found this to be a problem?  It looks like
> >> a bug in block because all inbound requests being prepared should
> >> have a timeout set, so block would be the place to fix it.
> >
> > Perhaps; what I found was that the value in rq->timeout was 0 coming
> > into this function and thus multiplying obviously has no effect.
> >
> 
> I think you are right. We hit this problem because we are doing:
> 
> scsi_request_fn -> blk_peek_request -> sd_prep_fn ->
> scsi_setup_flush_cmnd.
> 
> At this time request->timeout is zero so the multiplication does nothing. See
> how sd_setup_write_same_cmnd will set the request->timeout at this time.
> 
> Then in scsi_request_fn we do:
> 
> scsi_request_fn -> blk_start_request -> blk_add_timer.
> 
> At this time it will set the request->timeout if something like req block pc
> users (like scsi_execute() or block/scsi_ioctl.c) or the write same code
> mentioned above have not set the timeout.

I don't think this is a recent change. Prior to this commit, we were setting the timeout
value in this function; it just happened to be a different constant unrelated to the I/O
timeout.

K. Y
> 
>