linux-kernel - Re: [PATCH v7 03/17] scsi: ufs: implement scsi host timeout handler

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Date:	Tue, 8 Mar 2016 19:58:25 -0000
From:	ygardi@...eaurora.org
To:	"Hannes Reinecke" <hare@...e.de>
Cc:	"Yaniv Gardi" <ygardi@...eaurora.org>,
	james.bottomley@...senpartnership.com,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, santoshsy@...il.com,
	linux-scsi-owner@...r.kernel.org,
	"Gilad Broner" <gbroner@...eaurora.org>,
	"Vinayak Holikatti" <vinholikatti@...il.com>,
	"James E.J. Bottomley" <jbottomley@...n.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH v7 03/17] scsi: ufs: implement scsi host timeout handler

> On 03/08/2016 02:01 PM, Hannes Reinecke wrote:
>> On 03/08/2016 01:35 PM, Yaniv Gardi wrote:
>>> A race condition exists between request requeueing and scsi layer
>>> error handling:
>>> When UFS driver queuecommand returns a busy status for a request,
>>> it will be requeued and its tag will be freed and set to -1.
>>> At the same time it is possible that the request will timeout and
>>> scsi layer will start error handling for it. The scsi layer reuses
>>> the request and its tag to send error related commands to the device,
>>> however its tag is no longer valid.
>>> As this request was never really sent to the device, there is no
>>> point to start error handling with the device.
>>> Implement the scsi error handling timeout callback and bypass SCSI
>>> error handling for request that were not actually sent to the device.
>>> For such requests simply reset the block layer timer. Otherwise, let
>>> SCSI layer perform the usual error handling.
>>>
>>> Reviewed-by: Dolev Raviv <draviv@...eaurora.org>
>>> Signed-off-by: Gilad Broner <gbroner@...eaurora.org>
>>> Signed-off-by: Yaniv Gardi <ygardi@...eaurora.org>
>>>
>>> ---
>>>  drivers/scsi/ufs/ufshcd.c | 36 ++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 36 insertions(+)
>>>
>> Having a timeout handler is always a good idea, even though this
>> doesn't do anything here.
>> Are we sure that the requests will return eventually?
>> Does the UFS spec provide for a command abort?
>>
> In fact, looking at the UFS spec there _is_ a command abort.
> I would recommend implementing a task management request UPIO with
> type 'ABORT TASK' here for any task found to be pending.
> In the end, you might run into a _valid_ timeout, at which point you
> really want to abort the command...
>

but this is not what we'd like to achieve.
we don't want to abort a task that was not even dispatched to the UFS driver.
in those cases we need to re-queue the request and reset the timer.

Hannes, i appreciate your time, but I really don't understand why you
insist on coming up with suggestions, when we already implemented one that
is working.
more over, your solution doesn't fix the race condition which is the
reason for this patch.
as i don't have HW to test anything at the moment, I think it's better to
stick with this solution that also fix the BUG and also was verified and
tested.

I'd really appreciate your approval for this patch, but, as already said, 
I can not implement anything else as i can't test it, and also - your
suggestion will NOT fix the race condition.

i think we shouldn't block the entire 17 patches series because of this
patch. not to say - this patch is a BUG fix, so it must be included.

thanks,
Yaniv



> Cheers,
>
> Hannes-
> --
> Dr. Hannes Reinecke		   Teamlead Storage & Networking
> hare@...e.de			               +49 911 74053 688
> SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
> GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
> HRB 21284 (AG Nürnberg)
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