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

[Jens Axboe <axboe@kernel.dk>]

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