Re: [PATCHv4 0/3] scsi timeout handling updates

[Hannes Reinecke <hare@suse.de>]

On 11/29/18 6:20 PM, Keith Busch wrote:
> On Thu, Nov 29, 2018 at 06:11:59PM +0100, Christoph Hellwig wrote:
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index a82830f39933..d0ef540711c7 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -647,7 +647,7 @@ EXPORT_SYMBOL(blk_mq_complete_request);
>>>   
>>>   int blk_mq_request_started(struct request *rq)
>>>   {
>>> -	return blk_mq_rq_state(rq) != MQ_RQ_IDLE;
>>> +	return blk_mq_rq_state(rq) == MQ_RQ_IN_FLIGHT;
>>>   }
>>>   EXPORT_SYMBOL_GPL(blk_mq_request_started);
>>
>> Independ of this series this change looks like the right thing to do.
>> But this whole area is a mine field, so separate testing would be
>> very helpful.
>>
>> I also wonder why we even bother with the above helper, a direct
>> state comparism seems a lot more obvious to the reader.
> 
> I think it's just because blk_mq_rq_state() is a private interface. The
> enum mq_rq_state is defined under include/linux/, so it looks okay to
> make getting the state public too.
>   
>> Last but not least the blk_mq_request_started check in nbd
>> should probably be lifted into blk_mq_tag_to_rq while we're at it..
>>
>> As for the nvme issue - it seems to me like we need to decouple the
>> nvme loop frontend request from the target backend request.  In case
>> of a timeout/reset we'll complete struct request like all other nvme
>> transport drivers, but we leave the backend target state around, which
>> will be freed when it completes (or leaks when the completion is lost).
> 
> I don't think nvme's loop target should do anything to help a command
> complete. It shouldn't even implement a timeout for the same reason
> no stacking block driver implements these. If a request is stuck, the
> lowest level is the only driver that should have the responsibility to
> make it unstuck.
> 
Not quite.
You still need to be able to reset the controller, which you can't if 
you have to wait for the lowest level.

Cheers,

Hannes