Re: FAILED: patch "[PATCH] io_uring: ensure ctx->rings is stable for task work flags" failed to apply to 6.19-stable tree

[Jens Axboe <axboe@kernel.dk>]

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On 3/17/26 6:55 AM, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 6.19-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>.

Here's this one and the following one too, for 6.19.

-- 
Jens Axboe

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From 70710c220b58e84e08b924833d4caeb1d1500859 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Mon, 9 Mar 2026 14:35:49 -0600
Subject: [PATCH 2/2] io_uring/eventfd: use ctx->rings_rcu for flags checking

Commit 177c69432161f6e4bab07ccacf8a1748a6898a6b upstream.

Similarly to what commit e78f7b70e837 did for local task work additions,
use ->rings_rcu under RCU rather than dereference ->rings directly. See
that commit for more details.

Cc: stable@vger.kernel.org
Fixes: 79cfe9e59c2a ("io_uring/register: add IORING_REGISTER_RESIZE_RINGS")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 io_uring/eventfd.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/io_uring/eventfd.c b/io_uring/eventfd.c
index 78f8ab7db104..ab789e1ebe91 100644
--- a/io_uring/eventfd.c
+++ b/io_uring/eventfd.c
@@ -76,11 +76,15 @@ void io_eventfd_signal(struct io_ring_ctx *ctx, bool cqe_event)
 {
 	bool skip = false;
 	struct io_ev_fd *ev_fd;
-
-	if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
-		return;
+	struct io_rings *rings;
 
 	guard(rcu)();
+
+	rings = rcu_dereference(ctx->rings_rcu);
+	if (!rings)
+		return;
+	if (READ_ONCE(rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
+		return;
 	ev_fd = rcu_dereference(ctx->io_ev_fd);
 	/*
 	 * Check again if ev_fd exists in case an io_eventfd_unregister call
-- 
2.53.0


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From 8e746bd1dadaf93542ca59dc971b233332c46ca6 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Mon, 9 Mar 2026 14:21:37 -0600
Subject: [PATCH 1/2] io_uring: ensure ctx->rings is stable for task work flags
 manipulation

Commit 96189080265e6bb5dde3a4afbaf947af493e3f82 upstream.

If DEFER_TASKRUN | SETUP_TASKRUN is used and task work is added while
the ring is being resized, it's possible for the OR'ing of
IORING_SQ_TASKRUN to happen in the small window of swapping into the
new rings and the old rings being freed.

Prevent this by adding a 2nd ->rings pointer, ->rings_rcu, which is
protected by RCU. The task work flags manipulation is inside RCU
already, and if the resize ring freeing is done post an RCU synchronize,
then there's no need to add locking to the fast path of task work
additions.

Note: this is only done for DEFER_TASKRUN, as that's the only setup mode
that supports ring resizing. If this ever changes, then they too need to
use the io_ctx_mark_taskrun() helper.

Link: https://lore.kernel.org/io-uring/20260309062759.482210-1-naup96721@gmail.com/
Cc: stable@vger.kernel.org
Fixes: 79cfe9e59c2a ("io_uring/register: add IORING_REGISTER_RESIZE_RINGS")
Reported-by: Hao-Yu Yang <naup96721@gmail.com>
Suggested-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/io_uring_types.h |  1 +
 io_uring/io_uring.c            | 24 ++++++++++++++++++++++--
 io_uring/register.c            | 12 ++++++++++++
 3 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
index 4c9770536eb5..f3a8a1306cf4 100644
--- a/include/linux/io_uring_types.h
+++ b/include/linux/io_uring_types.h
@@ -371,6 +371,7 @@ struct io_ring_ctx {
 	 * regularly bounce b/w CPUs.
 	 */
 	struct {
+		struct io_rings	__rcu	*rings_rcu;
 		struct llist_head	work_llist;
 		struct llist_head	retry_llist;
 		unsigned long		check_cq;
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 63efd60829f3..5b7ebf143412 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1202,6 +1202,21 @@ void tctx_task_work(struct callback_head *cb)
 	WARN_ON_ONCE(ret);
 }
 
+/*
+ * Sets IORING_SQ_TASKRUN in the sq_flags shared with userspace, using the
+ * RCU protected rings pointer to be safe against concurrent ring resizing.
+ */
+static void io_ctx_mark_taskrun(struct io_ring_ctx *ctx)
+{
+	lockdep_assert_in_rcu_read_lock();
+
+	if (ctx->flags & IORING_SETUP_TASKRUN_FLAG) {
+		struct io_rings *rings = rcu_dereference(ctx->rings_rcu);
+
+		atomic_or(IORING_SQ_TASKRUN, &rings->sq_flags);
+	}
+}
+
 static void io_req_local_work_add(struct io_kiocb *req, unsigned flags)
 {
 	struct io_ring_ctx *ctx = req->ctx;
@@ -1256,8 +1271,7 @@ static void io_req_local_work_add(struct io_kiocb *req, unsigned flags)
 	 */
 
 	if (!head) {
-		if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
-			atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
+		io_ctx_mark_taskrun(ctx);
 		if (ctx->has_evfd)
 			io_eventfd_signal(ctx, false);
 	}
@@ -1281,6 +1295,10 @@ static void io_req_normal_work_add(struct io_kiocb *req)
 	if (!llist_add(&req->io_task_work.node, &tctx->task_list))
 		return;
 
+	/*
+	 * Doesn't need to use ->rings_rcu, as resizing isn't supported for
+	 * !DEFER_TASKRUN.
+	 */
 	if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
 		atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
 
@@ -2760,6 +2778,7 @@ static void io_rings_free(struct io_ring_ctx *ctx)
 	io_free_region(ctx->user, &ctx->sq_region);
 	io_free_region(ctx->user, &ctx->ring_region);
 	ctx->rings = NULL;
+	RCU_INIT_POINTER(ctx->rings_rcu, NULL);
 	ctx->sq_sqes = NULL;
 }
 
@@ -3389,6 +3408,7 @@ static __cold int io_allocate_scq_urings(struct io_ring_ctx *ctx,
 	if (ret)
 		return ret;
 	ctx->rings = rings = io_region_get_ptr(&ctx->ring_region);
+	rcu_assign_pointer(ctx->rings_rcu, rings);
 	if (!(ctx->flags & IORING_SETUP_NO_SQARRAY))
 		ctx->sq_array = (u32 *)((char *)rings + rl->sq_array_offset);
 
diff --git a/io_uring/register.c b/io_uring/register.c
index 12318c276068..ff9d75ffbd15 100644
--- a/io_uring/register.c
+++ b/io_uring/register.c
@@ -545,7 +545,15 @@ static int io_register_resize_rings(struct io_ring_ctx *ctx, void __user *arg)
 	ctx->sq_entries = p->sq_entries;
 	ctx->cq_entries = p->cq_entries;
 
+	/*
+	 * Just mark any flag we may have missed and that the application
+	 * should act on unconditionally. Worst case it'll be an extra
+	 * syscall.
+	 */
+	atomic_or(IORING_SQ_TASKRUN | IORING_SQ_NEED_WAKEUP, &n.rings->sq_flags);
 	ctx->rings = n.rings;
+	rcu_assign_pointer(ctx->rings_rcu, n.rings);
+
 	ctx->sq_sqes = n.sq_sqes;
 	swap_old(ctx, o, n, ring_region);
 	swap_old(ctx, o, n, sq_region);
@@ -554,6 +562,10 @@ static int io_register_resize_rings(struct io_ring_ctx *ctx, void __user *arg)
 out:
 	spin_unlock(&ctx->completion_lock);
 	mutex_unlock(&ctx->mmap_lock);
+
+	/* Wait for concurrent io_ctx_mark_taskrun() */
+	if (to_free == &o)
+		synchronize_rcu_expedited();
 	io_register_free_rings(ctx, to_free);
 
 	if (ctx->sq_data)
-- 
2.53.0