Re: [PATCH] io_uring: remove PF_EXITING checking in io_poll_rewait()

[Jens Axboe <axboe@kernel.dk>]

On 8/19/21 11:36 AM, Hao Xu wrote:
> 在 2021/8/20 上午1:29, Jens Axboe 写道:
>> On 8/19/21 11:26 AM, Hao Xu wrote:
>>> 在 2021/8/19 下午11:48, Jens Axboe 写道:
>>>> We have two checks of task->flags & PF_EXITING left:
>>>>
>>>> 1) In io_req_task_submit(), which is called in task_work and hence always
>>>>      in the context of the original task. That means that
>>>>      req->task == current, and hence checking ->flags is totally fine.
>>>>
>>>> 2) In io_poll_rewait(), where we need to stop re-arming poll to prevent
>>>>      it interfering with cancelation. Here, req->task is not necessarily
>>>>      current, and hence the check is racy. Use the ctx refs state instead
>>>>      to check if we need to cancel this request or not.
>>> Hi Jens,
>>> I saw cases that io_req_task_submit() and io_poll_rewait() in one
>>> function, why one is safe and the other one not? btw, it seems both two
>>> executes in task_work context..and task_work_add() may fail and then
>>> work goes to system_wq, is that case safe?
> I've got answer for the second question..
>>
>> io_req_task_submit() is guaranteed to be run in the task that is req->task,
>> io_poll_rewait() is not. The latter can get called from eg the poll
>> waitqueue handling, which is not run from the task in question.
> I only found io_poll_rewait() call in io_async_task_func() and
> io_poll_task_func(), both are in task_work

Yeah see followup, my information was outdated, we only do rewait from the
right context at this point. Hence the PF_EXITING check is actually fine.

-- 
Jens Axboe