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

[Hao Xu <haoxu@linux.alibaba.com>]

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
>