FAILED: patch "[PATCH] io_uring/tw: serialize ctx->retry_llist with ->uring_lock" failed to apply to 6.18-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.18-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-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 17666e2d7592c3e85260cafd3950121524acc2c5
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026051242-reproach-duplicity-8d39@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 17666e2d7592c3e85260cafd3950121524acc2c5 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Mon, 27 Apr 2026 14:29:02 -0600
Subject: [PATCH] io_uring/tw: serialize ctx->retry_llist with ->uring_lock

The DEFER_TASKRUN local task work paths all run under ctx->uring_lock,
which serializes them with each other and with the rest of the ring's
hot paths. io_move_task_work_from_local() is the exception - it's called
from io_ring_exit_work() on a kworker without holding the lock and from
the iopoll cancelation side right after dropping it.

->work_llist is fine with this, as it's only ever updated via the
expected paths. But the ->retry_llist is updated while runing, and hence
it could potentially race between normal task_work running and the
task-has-exited shutdown path.

Simply grab ->uring_lock while moving the local work to the fallback
list for exit purposes, which nicely serializes it across both the
normal additions and the exit prune path.

Cc: stable@vger.kernel.org
Fixes: f46b9cdb22f7 ("io_uring: limit local tw done")
Reported-by: Robert Femmer <robert.femmer@x41-dsec.de>
Reported-by: Christian Reitter <invd@inhq.net>
Reported-by: Michael Rodler <michael.rodler@x41-dsec.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/io_uring/tw.c b/io_uring/tw.c
index fdff81eebc95..023d5e6bc491 100644
--- a/io_uring/tw.c
+++ b/io_uring/tw.c
@@ -273,8 +273,18 @@ void io_req_task_work_add_remote(struct io_kiocb *req, unsigned flags)
 
 void __cold io_move_task_work_from_local(struct io_ring_ctx *ctx)
 {
-	struct llist_node *node = llist_del_all(&ctx->work_llist);
+	struct llist_node *node;
 
+	/*
+	 * Running the work items may utilize ->retry_llist as a means
+	 * for capping the number of task_work entries run at the same
+	 * time. But that list can potentially race with moving the work
+	 * from here, if the task is exiting. As any normal task_work
+	 * running holds ->uring_lock already, just guard this slow path
+	 * with ->uring_lock to avoid racing on ->retry_llist.
+	 */
+	guard(mutex)(&ctx->uring_lock);
+	node = llist_del_all(&ctx->work_llist);
 	__io_fallback_tw(node, false);
 	node = llist_del_all(&ctx->retry_llist);
 	__io_fallback_tw(node, false);