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

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

在 2021/8/20 上午1:36, Jens Axboe 写道:
> On 8/19/21 11:29 AM, Jens Axboe wrote:
>> 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?
>>
>> 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.
> 
> Pavel nudged me, and in the 5.15 branch we actually only do run rewait
> from the task itself. So this patch isn't needed, we can ignore it!
> Might just augment it with a comment, like it was done for submit.
saw this after my second email. cool!
>