Re: [PATCH v3] io_uring: fix possible deadlock in io_uring_poll

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

On 05/02/2021 08:34, Hao Xu wrote:
> This might happen if we do epoll_wait on a uring fd while reading/writing
> the former epoll fd in a sqe in the former uring instance.
> So let's don't flush cqring overflow list, just do a simple check.

I haven't tested it but tried out identical before.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Cc: stable@vger.kernel.org # 5.5+

> 
> Reported-by: Abaci <abaci@linux.alibaba.com>
> Fixes: 6c503150ae33 ("io_uring: patch up IOPOLL overflow_flush sync")
> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
>  fs/io_uring.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 38c6cbe1ab38..5f42ad6f2155 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -8718,8 +8718,21 @@ static __poll_t io_uring_poll(struct file *file, poll_table *wait)
>  	smp_rmb();
>  	if (!io_sqring_full(ctx))
>  		mask |= EPOLLOUT | EPOLLWRNORM;
> -	io_cqring_overflow_flush(ctx, false, NULL, NULL);
> -	if (io_cqring_events(ctx))
> +
> +	/*
> +	 * Don't flush cqring overflow list here, just do a simple check.
> +	 * Otherwise there could possible be ABBA deadlock:
> +	 *      CPU0                    CPU1
> +	 *      ----                    ----
> +	 * lock(&ctx->uring_lock);
> +	 *                              lock(&ep->mtx);
> +	 *                              lock(&ctx->uring_lock);
> +	 * lock(&ep->mtx);
> +	 *
> +	 * Users may get EPOLLIN meanwhile seeing nothing in cqring, this
> +	 * pushs them to do the flush.
> +	 */
> +	if (io_cqring_events(ctx) || test_bit(0, &ctx->cq_check_overflow))
>  		mask |= EPOLLIN | EPOLLRDNORM;
>  
>  	return mask;
> 

-- 
Pavel Begunkov