Re: [PATCH for-5.13] io_uring: maintain drain requests' logic

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

On 31/03/2021 10:01, Hao Xu wrote:
> Now that we have multishot poll requests, one sqe can emit multiple
> cqes. given below example:
>     sqe0(multishot poll)-->sqe1-->sqe2(drain req)
> sqe2 is designed to issue after sqe0 and sqe1 completed, but since sqe0
> is a multishot poll request, sqe2 may be issued after sqe0's event
> triggered twice before sqe1 completed. This isn't what users leverage
> drain requests for.
> Here a simple solution is to ignore all multishot poll cqes, which means
> drain requests  won't wait those request to be done.
> 
> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
>  fs/io_uring.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 513096759445..cd6d44cf5940 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -455,6 +455,7 @@ struct io_ring_ctx {
>  	struct callback_head		*exit_task_work;
>  
>  	struct wait_queue_head		hash_wait;
> +	unsigned                        multishot_cqes;
>  
>  	/* Keep this last, we don't need it for the fast path */
>  	struct work_struct		exit_work;
> @@ -1181,8 +1182,8 @@ static bool req_need_defer(struct io_kiocb *req, u32 seq)
>  	if (unlikely(req->flags & REQ_F_IO_DRAIN)) {
>  		struct io_ring_ctx *ctx = req->ctx;
>  
> -		return seq != ctx->cached_cq_tail
> -				+ READ_ONCE(ctx->cached_cq_overflow);
> +		return seq + ctx->multishot_cqes != ctx->cached_cq_tail
> +			+ READ_ONCE(ctx->cached_cq_overflow);
>  	}
>  
>  	return false;
> @@ -4897,6 +4898,7 @@ static bool io_poll_complete(struct io_kiocb *req, __poll_t mask, int error)
>  {
>  	struct io_ring_ctx *ctx = req->ctx;
>  	unsigned flags = IORING_CQE_F_MORE;
> +	bool multishot_poll = !(req->poll.events & EPOLLONESHOT);
>  
>  	if (!error && req->poll.canceled) {
>  		error = -ECANCELED;
> @@ -4911,6 +4913,9 @@ static bool io_poll_complete(struct io_kiocb *req, __poll_t mask, int error)
>  		req->poll.done = true;
>  		flags = 0;
>  	}
> +	if (multishot_poll)
> +		ctx->multishot_cqes++;
> +

We need to make sure we do that only for a non-final complete, i.e.
not killing request, otherwise it'll double account the last one.
E.g. is failed __io_cqring_fill_event() in io_poll_complete() fine?
Other places?

Btw, we can use some tests :)


>  	io_commit_cqring(ctx);
>  	return !(flags & IORING_CQE_F_MORE);
>  }
> 

-- 
Pavel Begunkov