Messages in this thread | | | Date | Fri, 06 Jun 2014 12:22:50 -0600 | From | Jens Axboe <> | Subject | Re: [PATCH 1/1] [SCSI] Fix a bug in deriving the FLUSH_TIMEOUT from the basic I/O timeout |
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On 2014-06-06 11:52, James Bottomley wrote: > On Fri, 2014-06-06 at 12:18 -0500, Mike Christie wrote: >> On 6/5/14, 9:53 PM, KY Srinivasan wrote: >>> >>> >>>> -----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. >>> >> >> Yeah, it looks like when 7e660100d85af860e7ad763202fff717adcdaacd was >> merged we were supposed to initialize it like in your patch in this thread. >> >> I guess we could do your patch in this thread, or if we want the block >> layer to initialize the timeout before the prep_fn callout is called >> then we would need to have the blk-flush.c code to that when it sets up >> the request. If we do the latter, do we want the discard and write same >> code to initialize the request's timeout before the prep_fn callout is >> called too? > > I looked through the call chain; it seems to be intentional behaviour on > the part of block. Just from an mq point of view, it would make better > code if we unconditionally initialised rq->timeout early and allowed > prep to modify it and then dumped the if(!req->timeout) in > blk_add_timer(), but it's a marginal if condition that would compile to > a conditional store on sensible architectures, so losing the conditional > probably isn't worth worrying about. > > Cc'd Jens for his opinion with the block patch
I just committed this one earlier today:
http://git.kernel.dk/?p=linux-block.git;a=commit;h=f6be4fb4bcb396fc3b1c134b7863351972de081f
since I ran into the same thing on nvme. Either approach is fine with me, as they both allow override of the timeout before insertion. But we've always done the rq->timeout = 0 init, so I think we should just reinstate that behavior.
-- Jens Axboe
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