Re: [PATCH] io_uring/eventfd: ensure io_eventfd_signal() defers another RCU period

[Prasanna Kumar T S M <ptsm@linux.microsoft.com>]

On 09-01-2025 05:12, Jens Axboe wrote:
> io_eventfd_do_signal() is invoked from an RCU callback, but when
> dropping the reference to the io_ev_fd, it calls io_eventfd_free()
> directly if the refcount drops to zero. This isn't correct, as any
> potential freeing of the io_ev_fd should be deferred another RCU grace
> period.
>
> Just call io_eventfd_put() rather than open-code the dec-and-test and
> free, which will correctly defer it another RCU grace period.
>
> Fixes: 21a091b970cd ("io_uring: signal registered eventfd to process deferred task work")
> Reported-by: Jann Horn <jannh@google.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>
> ---
>
> diff --git a/io_uring/eventfd.c b/io_uring/eventfd.c
> index fab936d31ba8..100d5da94cb9 100644
> --- a/io_uring/eventfd.c
> +++ b/io_uring/eventfd.c
> @@ -33,20 +33,18 @@ static void io_eventfd_free(struct rcu_head *rcu)
>   	kfree(ev_fd);
>   }
>   
> -static void io_eventfd_do_signal(struct rcu_head *rcu)
> +static void io_eventfd_put(struct io_ev_fd *ev_fd)
>   {
> -	struct io_ev_fd *ev_fd = container_of(rcu, struct io_ev_fd, rcu);
> -
> -	eventfd_signal_mask(ev_fd->cq_ev_fd, EPOLL_URING_WAKE);
> -
>   	if (refcount_dec_and_test(&ev_fd->refs))
> -		io_eventfd_free(rcu);
> +		call_rcu(&ev_fd->rcu, io_eventfd_free);
>   }
>   
> -static void io_eventfd_put(struct io_ev_fd *ev_fd)
> +static void io_eventfd_do_signal(struct rcu_head *rcu)
>   {
> -	if (refcount_dec_and_test(&ev_fd->refs))
> -		call_rcu(&ev_fd->rcu, io_eventfd_free);
> +	struct io_ev_fd *ev_fd = container_of(rcu, struct io_ev_fd, rcu);
> +
> +	eventfd_signal_mask(ev_fd->cq_ev_fd, EPOLL_URING_WAKE);
> +	io_eventfd_put(ev_fd);
>   }
>   
>   static void io_eventfd_release(struct io_ev_fd *ev_fd, bool put_ref)
>
Looks good to me.

Reviewed-by: Prasanna Kumar T S M <ptsm@linux.microsoft.com>