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

[ygardi@codeaurora.org]

Hello, Hannes,

Re-sending

thanks,
Yaniv

>> On 03/03/2016 05:10 PM, ygardi@codeaurora.org wrote:
>>>> On 03/01/2016 09:25 PM, ygardi@codeaurora.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 however its tag is no longer valid.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.
>> Oh, definitely agreed. The question is _where_.
>>
>>
>>> 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".
>> Well ... seeing it's your driver, I would've thought _you_ should know
>> ...
>>
>>> so, changing the already tested and working code, (not to return BUSY
>>> from
>>> queuecommand) is not a fix.
>> Hey, I did _not_ suggest not to retury BUSY from queuecommand.
>>
>> I was suggesting this patch:
>>
>> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
>> index 9c1b94b..b9295ad 100644
>> --- a/drivers/scsi/ufs/ufshcd.c
>> +++ b/drivers/scsi/ufs/ufshcd.c
>> @@ -1388,7 +1388,7 @@ static int ufshcd_queuecommand(struct Scsi_Host
>> *host, struct scsi_cmnd *cmd)
>>                 goto out;
>>         }
>>
>> -       err = ufshcd_hold(hba, true);
>> +       err = ufshcd_hold(hba, false);
>>         if (err) {
>>                 err = SCSI_MLQUEUE_HOST_BUSY;
>>                 clear_bit_unlock(tag, &hba->lrb_in_use);
>>
>> which, by reading the code, should be avoiding this issue.
>
>
> Hannes,
> we are not trying to avoid returning BUSY from queuecommand().
> On the contrary. By returning BUSY we actually re-queuing the request
> which is exactly what we need to do.
> your patch doesn't fix the race condition.
>
> thanks,
> Yaniv
>
>> I was just asking you if you could give this patch a spin and see if it
>> works. If not (for whatever reason) I'm happy to accept your patch.
>> But first I would like to have an explanation why the above would _not_
>> work.
>>
>> Unfortunately I don't have the hardware otherwise I'd be running the
>> tests myself.
>>
>> Cheers,
>>
>> Hannes
>> --
>> Dr. Hannes Reinecke		      zSeries & Storage
>> hare@suse.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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