Re: [PATCH 2/2] io_uring: use TWA_SIGNAL for task_work if the task isn't running

[peterz@infradead.org]

On Sat, Aug 08, 2020 at 12:34:39PM -0600, Jens Axboe wrote:
> An earlier commit:
> 
> b7db41c9e03b ("io_uring: fix regression with always ignoring signals in io_cqring_wait()")
> 
> ensured that we didn't get stuck waiting for eventfd reads when it's
> registered with the io_uring ring for event notification, but we still
> have a gap where the task can be waiting on other events in the kernel
> and need a bigger nudge to make forward progress.
> 
> Ensure that we use signaled notifications for a task that isn't currently
> running, to be certain the work is seen and processed immediately.
> 
> Cc: stable@vger.kernel.org # v5.7+
> Reported-by: Josef <josef.grieb@gmail.com>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  fs/io_uring.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index e9b27cdaa735..443eecdfeda9 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1712,21 +1712,27 @@ static int io_req_task_work_add(struct io_kiocb *req, struct callback_head *cb)
>  	struct io_ring_ctx *ctx = req->ctx;
>  	int ret, notify = TWA_RESUME;
>  
> +	ret = __task_work_add(tsk, cb);
> +	if (unlikely(ret))
> +		return ret;
> +
>  	/*
>  	 * SQPOLL kernel thread doesn't need notification, just a wakeup.
> -	 * If we're not using an eventfd, then TWA_RESUME is always fine,
> -	 * as we won't have dependencies between request completions for
> -	 * other kernel wait conditions.
> +	 * For any other work, use signaled wakeups if the task isn't
> +	 * running to avoid dependencies between tasks or threads. If
> +	 * the issuing task is currently waiting in the kernel on a thread,
> +	 * and same thread is waiting for a completion event, then we need
> +	 * to ensure that the issuing task processes task_work. TWA_SIGNAL
> +	 * is needed for that.
>  	 */
>  	if (ctx->flags & IORING_SETUP_SQPOLL)
>  		notify = 0;
> -	else if (ctx->cq_ev_fd)
> +	else if (READ_ONCE(tsk->state) != TASK_RUNNING)
>  		notify = TWA_SIGNAL;
>  
> -	ret = task_work_add(tsk, cb, notify);
> -	if (!ret)
> -		wake_up_process(tsk);
> -	return ret;
> +	__task_work_notify(tsk, notify);
> +	wake_up_process(tsk);
> +	return 0;
>  }

Wait.. so the only change here is that you look at tsk->state, _after_
doing __task_work_add(), but nothing, not the Changelog nor the comment
explains this.

So you're relying on __task_work_add() being an smp_mb() vs the add, and
you order this against the smp_mb() in set_current_state() ?

This really needs spelling out.