linux-kernel - Re: [PATCH v5 03/15] scsi: ufs: implement scsi host timeout handler

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Date:	Thu, 3 Mar 2016 09:10:16 -0000
From:	ygardi@...eaurora.org
To:	"Hannes Reinecke" <hare@...e.de>
Cc:	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 v5 03/15] scsi: ufs: implement scsi host timeout handler

> On 03/01/2016 09:25 PM, ygardi@...eaurora.org wrote:
>>> On 02/28/2016 09:32 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 however its tag is no longer valid.
>>> Hmm. How can the host return a 'busy' status for a request?
>>> From my understanding we have three possibilities:
>>>
>>> 1) queuecommand returns busy; however, that means that the command has
>>> never been send and this issue shouldn't occur
>>> 2) The command returns with BUSY status. But in this case it has
>>> already
>>> been returned, so there cannot be any timeout coming in.
>>> 3) The host receives a command with a tag which is already in-use.
>>> However, that should have been prevented by the block-layer, which
>>> really should ensure that this situation never happens.
>>>
>>> So either way I look at it, it really looks like a bug and adding a
>>> timeout handler will just paper over it.
>>> (Not that a timeout handler is a bad idea, in fact I'm convinced that
>>> you need one. Just not for this purpose.)
>>>
>>> So can you elaborate how this 'busy' status comes about?
>>> Is the command sent to the device?
>>>
>>> Cheers,
>>>
>>> Hannes
>>
>>
>> Hi Hannes,
>>
>> it's going to be a bit long :)
>> I think you are missing the point.
>> I will describe a race condition happened to us a while ago, that was
>> quite difficult to understand and fix.
>> So, this patch is not about the "busy" returning to the scsi dispatch
>> routine. it's about the abort triggered after 30 seconds.
>>
>> imagine a request being queued and sent to the scsi, and then to the
>> ufs.
>> a timer, initialized to 30 seconds start ticking.
>> but the request is never sent to the ufs device, as queuecommand()
>> returns
>> with "SCSI_MLQUEUE_HOST_BUSY"
>> by looking at the code, this could happen, for example:
>> 	err = ufshcd_hold(hba, true);
>> 	if (err) {
>> 		err = SCSI_MLQUEUE_HOST_BUSY;
>> 		goto out;
>> 	}
>>
> Uuhhh.
> You probably should not have pointed me to that piece of code ...
> open-coding loops in ufshcd_hold() ... shudder.
> (Did I ever review that one? Must've ...)
> _Anyway_: sleeping in queuecommand is always a bad idea, as then
> precisely those issues you've just described will happen.
>
> Couldn't you just call
> ufshcd_hold(hba, false)
> instead of
> ufshcd_hold(hba, true)
> ?
> The request will be requeued more-or-less immediately, avoiding the
> issue with timeout handler kicking in.
> And the queue will remain blocked until the ungate work item returns, at
> which point I/O submission will continue.
> As the request will be requeued to the head of the queue there won't be
> other I/O competing with tags, so it shouldn't have any adverse effects.
>
> Wouldn't that work?
>
> Cheers,
>
> Hannes

Hi Hannes

This is a bug, and it should be fixed.
if you choose to bypass it, by calling ufshcd_hold(hba, false), not only
the race condition is still there, and can pop-out at any other point in
the future, but also, not sure what are the consequences of
ufshcd_hold(hba, false) unstead of "true".
so, changing the already tested and working code, (not to return BUSY from
queuecommand) is not a fix.
I strongly recommend we upstream this race-condition fix.

thanks,
Yaniv



> --
> Dr. Hannes Reinecke		      zSeries & Storage
> hare@...e.de			      +49 911 74053 688
> SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
> GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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