[PATCH] io_uring/eventfd: reset deferred signal state

[PATCH] io_uring/eventfd: reset deferred signal state

[Yufan Chen]

From: Yufan Chen <ericterminal@gmail.com>

Recursive eventfd wakeups must defer io_uring eventfd signaling because
eventfd_signal_mask() rejects reentry from eventfd wakeup handlers. The
io_ev_fd ops bit tracks an outstanding deferred signal so that the same
rcu_head is not queued twice.

That bit is only set today. Once the first deferred callback runs, later
recursive notifications still see the bit set and skip queueing another
deferred signal. This can leave new completions without a matching eventfd
wake after the first recursive deferral.

Clear the pending bit before issuing the deferred signal. If the wakeup
path recurses while the callback runs, a new signal can be queued for the
next RCU grace period while the current callback keeps its reference until
it returns.

Signed-off-by: Yufan Chen <ericterminal@gmail.com>
---
 io_uring/eventfd.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/io_uring/eventfd.c b/io_uring/eventfd.c
index 3da028500f7..d656cc2a0b9 100644
--- a/io_uring/eventfd.c
+++ b/io_uring/eventfd.c
@@ -43,6 +43,7 @@ static void io_eventfd_do_signal(struct rcu_head *rcu)
 {
 	struct io_ev_fd *ev_fd = container_of(rcu, struct io_ev_fd, rcu);
 
+	atomic_andnot(BIT(IO_EVENTFD_OP_SIGNAL_BIT), &ev_fd->ops);
 	eventfd_signal_mask(ev_fd->cq_ev_fd, EPOLL_URING_WAKE);
 	io_eventfd_put(ev_fd);
 }
-- 
2.47.3

Re: [PATCH] io_uring/eventfd: reset deferred signal state

[Jens Axboe]

On Mon, 04 May 2026 01:57:10 +0800, Yufan Chen wrote:
> Recursive eventfd wakeups must defer io_uring eventfd signaling because
> eventfd_signal_mask() rejects reentry from eventfd wakeup handlers. The
> io_ev_fd ops bit tracks an outstanding deferred signal so that the same
> rcu_head is not queued twice.
> 
> That bit is only set today. Once the first deferred callback runs, later
> recursive notifications still see the bit set and skip queueing another
> deferred signal. This can leave new completions without a matching eventfd
> wake after the first recursive deferral.
> 
> [...]

Applied, thanks!

[1/1] io_uring/eventfd: reset deferred signal state
      commit: 0f32b6daa03ab8c1831f650d617f59447af31172

Best regards,
-- 
Jens Axboe