Re: [PATCH v3] io_uring/poll: fix multishot recv missing EOF on wakeup race

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/17/26 02:17, Jens Axboe wrote:
> When a socket send and shutdown() happen back-to-back, both fire
> wake-ups before the receiver's task_work has a chance to run. The first
> wake gets poll ownership (poll_refs=1), and the second bumps it to 2.
> When io_poll_check_events() runs, it calls io_poll_issue() which does a
> recv that reads the data and returns IOU_RETRY. The loop then drains all
> accumulated refs (atomic_sub_return(2) -> 0) and exits, even though only
> the first event was consumed. Since the shutdown is a persistent state
> change, no further wakeups will happen, and the multishot recv can hang
> forever.
> 
> Check specifically for HUP in the poll loop, and ensure that another
> loop is done to check for status if more than a single poll activation
> is pending. This ensures we don't lose the shutdown event.

Sounds fine with comments below.

Btw, did you look into whether it's a INQ issue? Polling expects
multishots to handle all those conditions, which usually goes in a
form of:

while (1) {
	ret = do_IO();
	if (ret == -EAGAIN)
		goto continue_poll;
	if (ret < 0)
		goto fail;
	if (ret == 0)
		goto terminate_req;
	...
	// partial progress, try again
}

and recv was following this pattern before, but maybe it's sth
like recv() returning some bytes, inq rightfully saying that there
are no more bytes left but forgets to check for terminators like
shutdown.

> Cc: stable@vger.kernel.org
> Fixes: dbc2564cfe0f ("io_uring: let fast poll support multishot")
> Reported-by: Francis Brosseau <francis@malagauche.com>
> Link: https://github.com/axboe/liburing/issues/1549
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> V3: split mshot and !mshot cases, and simply use the number of refs
>      gotten in the beginning for gating retry. if one is dropped when
>      we want to retry, we'll loop again as we'd still have remaining
>      refs.
> 
> diff --git a/io_uring/poll.c b/io_uring/poll.c
> index aac4b3b881fb..a264d73a8cbd 100644
> --- a/io_uring/poll.c
> +++ b/io_uring/poll.c
> @@ -228,6 +228,19 @@ static inline void io_poll_execute(struct io_kiocb *req, int res)
>   		__io_poll_execute(req, res);
>   }
>   
> +static inline void io_mshot_check_retry(struct io_kiocb *req, int *v)
> +{
> +	/*
> +	 * Release all references, retry if someone tried to restart
> +	 * task_work while we were executing it.
> +	 */

This comment belongs to the atomic sub, not masking.

> +	*v &= IO_POLL_REF_MASK;

nit: seems like you can just do that inside the
"if (unlikely(v != 1)) { ... }" block.

> +
> +	/* multiple refs and HUP, ensure we loop once more */
> +	if ((req->cqe.res & (POLLHUP | POLLRDHUP)) && *v != 1)
> +		(*v)--;
> +}
> +
>   /*
>    * All poll tw should go through this. Checks for poll events, manages
>    * references, does rewait, etc.
> @@ -303,6 +316,7 @@ static int io_poll_check_events(struct io_kiocb *req, io_tw_token_t tw)
>   				io_req_set_res(req, mask, 0);
>   				return IOU_POLL_REMOVE_POLL_USE_RES;
>   			}
> +			v &= IO_POLL_REF_MASK;
>   		} else {
>   			int ret = io_poll_issue(req, tw);
>   
> @@ -312,16 +326,11 @@ static int io_poll_check_events(struct io_kiocb *req, io_tw_token_t tw)
>   				return IOU_POLL_REQUEUE;
>   			if (ret != IOU_RETRY && ret < 0)
>   				return ret;
> +			io_mshot_check_retry(req, &v);

Should go before io_poll_issue(), req->cqe.res might already be
invalid.

-- 
Pavel Begunkov