Re: [PATCH V2 7/8] nvme: use blk_mq_queue_tag_inflight_iter

["jianchao.wang" <jianchao.w.wang@oracle.com>]

On 3/27/19 10:15 AM, Keith Busch wrote:
> On Wed, Mar 27, 2019 at 10:03:26AM +0800, jianchao.wang wrote:
>> Hi Keith
>>
>> On 3/27/19 7:57 AM, Keith Busch wrote:
>>> On Mon, Mar 25, 2019 at 08:05:53PM -0700, jianchao.wang wrote:
>>>> What if there used to be a io scheduler and leave some stale requests of sched tags ?
>>>> Or the nr_hw_queues was decreased and leave the hctx->fq->flush_rq ?
>>>
>>> Requests internally queued in scheduler or block layer are not eligible
>>> for the nvme driver's iterator callback. We only use it to reclaim
>>> dispatched requests that the target can't return, which only applies to
>>> requests that must have a valid rq->tag value from hctx->tags.
>>>  
>>>> The stable request could be some tings freed and used
>>>> by others and the state field happen to be overwritten to non-zero...
>>>
>>> I am not sure I follow what this means. At least for nvme, every queue
>>> sharing the same tagset is quiesced and frozen, there should be no
>>> request state in flux at the time we iterate.
>>>
>>
>> In nvme_dev_disable, when we try to reclaim the in-flight requests with blk_mq_tagset_busy_iter,
>> the request_queues are quiesced but just start-freeze.
>> We will try to _drain_ the in-flight requests for the _shutdown_ case when controller is not dead.
>> For the reset case, there still could be someone escapes the checking of queue freezing and enters
>> blk_mq_make_request and tries to allocate tag, then we may get,
>>
>> generic_make_request        nvme_dev_disable
>>  -> blk_queue_enter              
>>                               -> nvme_start_freeze (just start freeze, no drain)
>>                               -> nvme_stop_queues
>>  -> blk_mq_make_request
>>   - > blk_mq_get_request      -> blk_mq_tagset_busy_iter
>>      -> blk_mq_get_tag
>>                       　　        -> bt_tags_for_each
>>                          　　　       -> bt_tags_iter
>>                               　　       -> rq = tags->rqs[] ---> [1]
>>      -> blk_mq_rq_ctx_init
>>        -> data->hctx->tags->rqs[rq->tag] = rq;
>>
>> The rq got on position [1] could be a stale request that has been freed due to,
>> 1. a hctx->fq.flush_rq of dead request_queue that shares the same tagset
>> 2. a removed io scheduler's sched request
>>
>> And this stale request may have been used by others and the request->state is changed to a non-zero
>> value and passes the checking of blk_mq_request_started and then it will be handled by nvme_cancel_request.
> 
> How is that request state going to be anyting other than IDLE? A freed
> request state is IDLE, and continues to be IDLE until dispatched. But
> dispatch is blocked for the entire tagset, so request states can't be
> started during an nvme reset.
> 

As the comment above, the stable request maybe something that has been freed due to following case,
1. a hctx->fq.flush_rq of dead request_queue that shares the same tagset
2. a removed io scheduler's sched request
and this freed request could be allocated by others which may change the field of request->state.

Thanks
Jianchao