Re: [PATCH 3/3] io_uring: tweak io_req_task_work_add

[Pavel Begunkov <asml.silence@gmail.com>]

On 6/30/21 10:45 PM, Jens Axboe wrote:
> On 6/30/21 3:38 PM, Pavel Begunkov wrote:
>> On 6/30/21 10:22 PM, Jens Axboe wrote:
>>> On 6/30/21 3:19 PM, Pavel Begunkov wrote:
>>>> On 6/30/21 10:17 PM, Jens Axboe wrote:
>>>>> On 6/30/21 2:54 PM, Pavel Begunkov wrote:
>>>>>> Whenever possible we don't want to fallback a request. task_work_add()
>>>>>> will be fine if the task is exiting, so don't check for PF_EXITING,
>>>>>> there is anyway only a relatively small gap between setting the flag
>>>>>> and doing the final task_work_run().
>>>>>>
>>>>>> Also add likely for the hot path.
>>>>>
>>>>> I'm not a huge fan of likely/unlikely, and in particular constructs like:
>>>>>
>>>>>> -	if (test_bit(0, &tctx->task_state) ||
>>>>>> +	if (likely(test_bit(0, &tctx->task_state)) ||
>>>>>>  	    test_and_set_bit(0, &tctx->task_state))
>>>>>>  		return 0;
>>>>>
>>>>> where the state is combined. In any case, it should be a separate
>>>>> change. If there's an "Also" paragraph in a patch, then that's also
>>>>> usually a good clue that that particular change should've been
>>>>> separate :-)
>>>>
>>>> Not sure what's wrong with likely above, but how about drop
>>>> this one then?
>>>
>>> Yep I did - we can do the exiting change separately, the commit message
>>
>> I think 1-2 is good enough for 5.14, I'll just send it for-next
>>
>>> just needs to be clarified a bit on why it's ok to do now. And that
>>
>> It should have been ok to do before those 2 patches, but
>> haven't tracked where it lost actuality.
> 
> Right, I was thinking it was related to the swapping of the signal
> exit and task work run ordering. But didn't look that far yet...

commit 6200b0ae4ea28a4bfd8eb434e33e6201b7a6a282
Author: Jens Axboe <axboe@kernel.dk>
Date:   Sun Sep 13 14:38:30 2020 -0600

    io_uring: don't run task work on an exiting task

Came from there, times where there still was io_uring_flush()
and no from do_exit() cancellations

-- 
Pavel Begunkov