Re: [PATCH 1/2] nvme: pci: simplify timeout handling

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

Hi ming

On 04/29/2018 09:36 AM, Ming Lei wrote:
> On Sun, Apr 29, 2018 at 6:27 AM, Ming Lei <tom.leiming@gmail.com> wrote:
>> On Sun, Apr 29, 2018 at 5:57 AM, Ming Lei <tom.leiming@gmail.com> wrote:
>>> On Sat, Apr 28, 2018 at 10:00 PM, jianchao.wang
>>> <jianchao.w.wang@oracle.com> wrote:
>>>> Hi ming
>>>>
>>>> On 04/27/2018 10:57 PM, Ming Lei wrote:
>>>>> I may not understand your point, once blk_sync_queue() returns, the
>>>>> timer itself is deactivated, meantime the synced .nvme_timeout() only
>>>>> returns EH_NOT_HANDLED before the deactivation.
>>>>>
>>>>> That means this timer won't be expired any more, so could you explain
>>>>> a bit why timeout can come again after blk_sync_queue() returns
>>>>
>>>> Please consider the following case:
>>>>
>>>> blk_sync_queue
>>>>   -> del_timer_sync
>>>>                           blk_mq_timeout_work
>>>>                             -> blk_mq_check_expired // return the timeout value
>>>>                             -> blk_mq_terninate_expired
>>>>                               -> .timeout //return EH_NOT_HANDLED
>>>>                             -> mod_timer // setup the timer again based on the result of blk_mq_check_expired
>>>>   -> cancel_work_sync
>>>> So after the blk_sync_queue, the timer may come back again, then the timeout work.
>>>
>>> OK, I was trying to avoid to use blk_abort_request(), but looks we
>>> may have to depend on it or similar way.
>>>
>>> BTW, that means blk_sync_queue() has been broken, even though the uses
>>> in blk_cleanup_queue().
>>>
>>> Another approach is to introduce one perpcu_ref of
>>> 'q->timeout_usage_counter' for
>>> syncing timeout, seems a bit over-kill too, but simpler in both theory
>>> and implement.
>>
>> Or one timout_mutex is enough.
> 
> Turns out it is SRCU.
> 
after split the timeout path into timer and workqueue two parts, if we don't drain the in-flight requests, the request_queue->timeout and the timeout work look like an issue of 'chicken and egg'.
how about introduce a flag to disable triggering of timeout work ?


Thanks
Jianchao