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

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

[Jens Axboe]

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)

-- 
Jens Axboe

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

[lizetao]

Hi,
> -----Original Message-----
> From: Jens Axboe <axboe@kernel.dk>
> Sent: Thursday, January 9, 2025 7:42 AM
> To: io-uring <io-uring@vger.kernel.org>
> Subject: [PATCH] io_uring/eventfd: ensure io_eventfd_signal() defers another
> RCU period
> 
> 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)
> 
> --
> Jens Axboe
> 
> 

Tested-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Li Zetao<lizetao1@huawei.com>

---
Li Zetao

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

[Prasanna Kumar T S M]

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>