Re: FAILED: patch "[PATCH] io_uring/eventfd: ensure io_eventfd_signal() defers another" failed to apply to 6.6-stable tree

[Jens Axboe <axboe@kernel.dk>]

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On 1/12/25 2:16 AM, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 6.6-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.
> 
> To reproduce the conflict and resubmit, you may use the following commands:
> 
> git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
> git checkout FETCH_HEAD
> git cherry-pick -x c9a40292a44e78f71258b8522655bffaf5753bdb
> # <resolve conflicts, build, test, etc.>
> git commit -s
> git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2025011245-hardy-tiptop-229e@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Here's the 6.6 version.

-- 
Jens Axboe

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From 981586dd7b85fc424d59be8414255ad46251958c Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Wed, 8 Jan 2025 11:16:13 -0700
Subject: [PATCH] io_uring/eventfd: ensure io_eventfd_signal() defers another
 RCU period

Commit c9a40292a44e78f71258b8522655bffaf5753bdb upstream.

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>
---
 io_uring/io_uring.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 0122f220ef0d..c7198fbcf734 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -537,6 +537,13 @@ static __cold void io_queue_deferred(struct io_ring_ctx *ctx)
 	}
 }
 
+static void io_eventfd_free(struct rcu_head *rcu)
+{
+	struct io_ev_fd *ev_fd = container_of(rcu, struct io_ev_fd, rcu);
+
+	eventfd_ctx_put(ev_fd->cq_ev_fd);
+	kfree(ev_fd);
+}
 
 static void io_eventfd_ops(struct rcu_head *rcu)
 {
@@ -550,10 +557,8 @@ static void io_eventfd_ops(struct rcu_head *rcu)
 	 * ordering in a race but if references are 0 we know we have to free
 	 * it regardless.
 	 */
-	if (atomic_dec_and_test(&ev_fd->refs)) {
-		eventfd_ctx_put(ev_fd->cq_ev_fd);
-		kfree(ev_fd);
-	}
+	if (atomic_dec_and_test(&ev_fd->refs))
+		call_rcu(&ev_fd->rcu, io_eventfd_free);
 }
 
 static void io_eventfd_signal(struct io_ring_ctx *ctx)
-- 
2.47.1