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

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Date:	Fri, 06 Jun 2014 12:18:57 -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 6/5/14, 9:53 PM, KY Srinivasan wrote:
>
>
>> -----Original Message-----
>> From: Mike Christie [mailto:michaelc@...wisc.edu]
>> Sent: Thursday, June 5, 2014 6:33 PM
>> To: KY Srinivasan
>> Cc: James Bottomley; 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 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.
>
> 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?

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