Re: [PATCH 2/6] nvme-pci: fix the freeze and quiesce for shutdown and reset case

[jianchao.wang]

Hi Keith

Sorry for bothering you again. ;)

There is a dangerous scenario which caused by nvme_wait_freeze in nvme_reset_work.
please consider it.

nvme_reset_work
  -> nvme_start_queues
  -> nvme_wait_freeze

if the controller no response, we have to rely on the timeout path.
there are issues below:
nvme_dev_disable need to be invoked.
nvme_dev_disable will quiesce queues, cancel and requeue and outstanding requests.
nvme_reset_work will hang at nvme_wait_freeze

if we set NVME_REQ_CANCELLED and return BLK_EH_HANDLED as the RESETTING case,
nvme_reset_work will hang forever, because no one could complete the entered requests.

if we invoke nvme_reset_ctrl after modify the state machine to be able to change to RESETTING
to RECONNECTING and queue reset_work, we still cannot move things forward, because the reset_work
is being executed.

if we use nvme_wait_freeze_timeout in nvme_reset_work, unfreeze and return if expires. But the 
timeout value is tricky..

Maybe we could use blk_set_preempt_only which is also gate on blk_queue_enter.
We needn't to drain the queue for it. It is lightweight.
And nvme needn't worry about the full queue prevent admin requests from being submitted.

Looking forward your precious advice.

Sincerely
Jianchao 


On 02/08/2018 09:40 AM, jianchao.wang wrote:
> Hi Keith
> 
> 
> Really thanks for your your precious time and kindly directive.
> That's really appreciated. :)
> 
> On 02/08/2018 12:13 AM, Keith Busch wrote:
>> On Wed, Feb 07, 2018 at 10:13:51AM +0800, jianchao.wang wrote:
>>> What's the difference ? Can you please point out.
>>> I have shared my understanding below.
>>> But actually, I don't get the point what's the difference you said.
>>
>> It sounds like you have all the pieces. Just keep this in mind: we don't
>> want to fail IO if we can prevent it.
>>
> Yes, absolutely.
> 
>> A request is allocated from an hctx pool of tags. Once the request is
>> allocated, it is permently tied to that hctx because that's where its
>> tag came from. If that hctx becomes invalid, the request has to be ended
>> with an error, and we can't do anything about that[*].
>>
>> Prior to a reset, we currently halt new requests from being allocated by
>> freezing the request queues. We unfreeze the queues after the new state
>> of the hctx's is established. This way all IO requests that were gating
>> on the unfreeze are guaranteed to enter into a valid context.
>>
>> You are proposing to skip freeze on a reset. New requests will then be
>> allocated before we've established the hctx map. Any request allocated
>> will have to be terminated in failure if the hctx is no longer valid
>> once the reset completes.
> Yes, if any previous hctx doesn't come back, the requests on that hctx
> will be drained with BLK_STS_IOERR.
> __blk_mq_update_nr_hw_queues
>   -> blk_mq_freeze_queue
>     -> blk_freeze_queue
>       -> blk_mq_freeze_queue_wait 
> But the nvmeq's cq_vector is -1.
>  
>> Yes, it's entirely possible today a request allocated prior to the reset
>> may need to be terminated after the reset. There's nothing we can do
>> about those except end them in failure, but we can prevent new ones from
>> sharing the same fate. You are removing that prevention, and that's what
>> I am complaining about.
> 
> Thanks again for your precious time to detail this.
> So I got what you concern about is that this patch doesn't freeze the queue for reset case
> any more. And there maybe new requests enter, which will be failed when the associated
> hctx doesn't come back during reset procedure. And this should be avoided.
> 
> I will change this in next V3 version.
> 
> 
>>  * Future consideration: we recently obtained a way to "steal" bios that
>> looks like it may be used to back out certain types of requests and let
>> the bio create a new one.
>>
> Yeah, that will be a great idea to reduce the loss when hctx is gone.
> 
> Sincerely
> Jianchao
> 
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