Re: [PATCH 4/4] io_uring: flush task work before waiting for ring exit

[Jens Axboe <axboe@kernel.dk>]

On 4/8/20 1:17 PM, Oleg Nesterov wrote:
> On 04/08, Jens Axboe wrote:
>>
>> Here's some more data. I added a WARN_ON_ONCE() for task->flags &
>> PF_EXITING on task_work_add() success, and it triggers with the
>> following backtrace:
> ...
>> which means that we've successfully added the task_work while the
>> process is exiting.
> 
> but this is fine, task_work_add(task) can succeed if task->flags & EXITING.
> 
> task_work_add(task, work) should only fail if this "task" has already passed
> exit_task_work(). Because if this task has already passed exit_task_work(),
> nothing else can flush this work and call work->func().

So the question remains, we basically have this:

A			B
task_work_run(tsk)
			task_work_add(tsk, io_poll_task_func())
process cbs
wait_for_completion()

with the last wait needing to flush the work added on the B side, since
that isn't part of the initial list.

I don't I can fully close that race _without_ re-running task work
there. Could do something ala:

A			B
mark context "dead"
task_work_run(tsk)
			if (context dead)
				task_work_add(helper, io_poll_task_func())
			else
				task_work_add(tsk, io_poll_task_func())
process cbs
wait_for_completion()

which would do the trick, but I still need to flush work after having
marked the context dead.

-- 
Jens Axboe