Re: [PATCH] io_uring: fix req cannot arm poll after polled

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

On 30/06/2020 15:41, Xuan Zhuo wrote:
> For example, there are multiple sqes recv with the same connection.
> When there is no data in the connection, the reqs of these sqes will
> be armed poll. Then if only a little data is received, only one req
> receives the data, and the other reqs get EAGAIN again. However,
> due to this flags REQ_F_POLLED, these reqs cannot enter the
> io_arm_poll_handler function. These reqs will be put into wq by
> io_queue_async_work, and the flags passed by io_wqe_worker when recv
> is called are BLOCK, which may make io_wqe_worker enter schedule in the
> network protocol stack. When the main process of io_uring exits,
> these io_wqe_workers still cannot exit. The connection will not be
> actively released until the connection is closed by the peer.

It's a problem unrelated to polling, though it may be a nice optimisation.
E.g. requests submitted with IOSQE_ASYNC will always get into io-wq.

Have you seen it yourself? When, io_uring is going away, it calls
io_wq_cancel_all(), which do send_sig(SIGINT) for all its workers. The
question is why this doesn't halt inflight send/recv down in the network
stack?

> 
> So we should allow req to arm poll again.
> 
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>  fs/io_uring.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index e507737..a309832 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -4406,7 +4406,7 @@ static bool io_arm_poll_handler(struct io_kiocb *req)
>  
>  	if (!req->file || !file_can_poll(req->file))
>  		return false;
> -	if (req->flags & (REQ_F_MUST_PUNT | REQ_F_POLLED))
> +	if (req->flags & REQ_F_MUST_PUNT)

You have a bit outdated sources.

>  		return false;
>  	if (!def->pollin && !def->pollout)
>  		return false;
> 

-- 
Pavel Begunkov