FAILED: patch "[PATCH] ALSA: core: Serialize deferred fasync state checks" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-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 <[email protected]>.

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-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 5337213381df578058e2e41da93cbd0e4639935f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026051255-pushup-cheek-6781@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 5337213381df578058e2e41da93cbd0e4639935f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?C=C3=A1ssio=20Gabriel?= <[email protected]>
Date: Wed, 6 May 2026 00:34:47 -0300
Subject: [PATCH] ALSA: core: Serialize deferred fasync state checks
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

snd_fasync_helper() updates fasync->on under snd_fasync_lock, and snd_fasync_work_fn() now also evaluates fasync->on under the same lock. snd_kill_fasync() still tests the flag before taking the lock, leaving an unsynchronized read against FASYNC enable/disable updates.

Move the enabled-state check into the locked section.

Also clear fasync->on under snd_fasync_lock in snd_fasync_free() before unlinking the pending entry. Together with the locked sender-side check, this publishes teardown before flushing the deferred work and prevents a racing sender from requeueing the entry after free has started.

index 5aca09edf971..833124c8e4fa 100644
@@ -148,9 +148,11 @@ EXPORT_SYMBOL_GPL(snd_fasync_helper);
 
 void snd_kill_fasync(struct snd_fasync *fasync, int signal, int poll)
 {
-	if (!fasync || !fasync->on)
+	if (!fasync)
 		return;
 	guard(spinlock_irqsave)(&snd_fasync_lock);
+	if (!fasync->on)
+		return;
 	fasync->signal = signal;
 	fasync->poll = poll;
 	list_move(&fasync->list, &snd_fasync_list);

@@ -163,8 +165,10 @@ void snd_fasync_free(struct snd_fasync *fasync)
 	if (!fasync)
 		return;
 
-	scoped_guard(spinlock_irq, &snd_fasync_lock)
+	scoped_guard(spinlock_irq, &snd_fasync_lock) {
+		fasync->on = 0;
 		list_del_init(&fasync->list);
+	}
 
 	flush_work(&snd_fasync_work);
 	kfree(fasync);

From: Cássio Gabriel <[email protected]>

[ Upstream commit 5337213381df578058e2e41da93cbd0e4639935f ]

snd_fasync_helper() updates fasync->on under snd_fasync_lock, and snd_fasync_work_fn() now also evaluates fasync->on under the same lock. snd_kill_fasync() still tests the flag before taking the lock, leaving an unsynchronized read against FASYNC enable/disable updates.

Move the enabled-state check into the locked section.

Also clear fasync->on under snd_fasync_lock in snd_fasync_free() before unlinking the pending entry. Together with the locked sender-side check, this publishes teardown before flushing the deferred work and prevents a racing sender from requeueing the entry after free has started.

---
 sound/core/misc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

index d32a19976a2b9..40dd3a23dac02 100644
@@ -219,9 +219,13 @@ void snd_kill_fasync(struct snd_fasync *fasync, int signal, int poll)
 {
 	unsigned long flags;
 
-	if (!fasync || !fasync->on)
+	if (!fasync)
 		return;
 	spin_lock_irqsave(&snd_fasync_lock, flags);
+	if (!fasync->on) {
+		spin_unlock_irqrestore(&snd_fasync_lock, flags);
+		return;
+	}
 	fasync->signal = signal;
 	fasync->poll = poll;
 	list_move(&fasync->list, &snd_fasync_list);

@@ -234,7 +238,9 @@ void snd_fasync_free(struct snd_fasync *fasync)
 {
 	if (!fasync)
 		return;
+	spin_lock_irq(&snd_fasync_lock);
 	fasync->on = 0;
+	spin_unlock_irq(&snd_fasync_lock);
 	flush_work(&snd_fasync_work);
 	kfree(fasync);
 }

2.53.0