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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe