linux-kernel - Re: [PATCH 1/1] [SCSI] Fix a bug in deriving the FLUSH

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Date:	Thu, 05 Jun 2014 20:32:37 -0500
From:	Mike Christie <michaelc@...wisc.edu>
To:	KY Srinivasan <kys@...rosoft.com>
CC:	James Bottomley <jbottomley@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"ohering@...e.com" <ohering@...e.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"jasowang@...hat.com" <jasowang@...hat.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@...allels.com]
>> Sent: Wednesday, June 4, 2014 10:02 AM
>> To: KY Srinivasan
>> Cc: linux-kernel@...r.kernel.org; apw@...onical.com;
>> devel@...uxdriverproject.org; hch@...radead.org; linux-
>> scsi@...r.kernel.org; ohering@...e.com; gregkh@...uxfoundation.org;
>> jasowang@...hat.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@...rosoft.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.



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