Re: [PATCH] io_uring: fix readiness race with poll based retry

[Jens Axboe <axboe@kernel.dk>]

On 5/28/20 7:51 PM, Jens Axboe wrote:
> The poll based retry handler uses the same logic as the normal poll
> requests, but the latter triggers a completion if we hit the slim
> race of:
> 
> 1a) data/space isn't available
> 2a) data/space becomes available
> 1b) arm poll handler (returns success, callback not armed)
> 
> This isn't the case for the task_work based retry, where we need to
> take action if the event triggered in the short time between trying
> and arming the poll handler.
> 
> Catch this case in __io_arm_poll_handler(), and queue the task_work
> upfront instead of depending on the waitq handler triggering it. The
> latter isn't armed at this point.

Disregard this one, I don't think this race exists. If we hit the
poll->head != NULL case, then we definitely added the waitq. And if
the list is empty once we're under the lock, we already triggered
the callback and queued the task_work.

-- 
Jens Axboe