[PATCH] ALSA: control_led: Fix deadlock between sysfs writes and card disconnect

[PATCH] ALSA: control_led: Fix deadlock between sysfs writes and card disconnect

[Ali Ahmet Memiş]

snd_ctl_led_disconnect() removes the ctl-led sysfs files while
holding card->controls_rwsem. device_unregister() waits for running
sysfs callbacks which can deadlock when a concurrent write causes
the attach or detach callback to wait for controls_rwsem.

Break kernfs active protection in the attach, detach and list
callbacks and hold a module reference instead.

Fixes: a135dfb5de15 ("ALSA: led control - add sysfs kcontrol LED marking layer")
Signed-off-by: Ali Ahmet Memiş <aliamemis@disroot.org>
---
 sound/core/control_led.c | 62 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 57 insertions(+), 5 deletions(-)

diff --git a/sound/core/control_led.c b/sound/core/control_led.c
index ec0e6c7ad..501387287 100644
--- a/sound/core/control_led.c
+++ b/sound/core/control_led.c
@@ -563,12 +563,43 @@ static ssize_t set_led_id(struct snd_ctl_led_card *led_card, const char *buf, si
 	return count;
 }
 
+/*
+ * snd_ctl_led_disconnect() removes these files with card->controls_rwsem
+ * held. so don't let the removal wait for a callback that takes it.
+ */
+static struct kernfs_node *snd_ctl_led_sysfs_enter(struct device *dev,
+						   struct device_attribute *attr)
+{
+	struct kernfs_node *kn;
+
+	if (!try_module_get(THIS_MODULE))
+		return NULL;
+	kn = sysfs_break_active_protection(&dev->kobj, &attr->attr);
+	if (!kn)
+		module_put(THIS_MODULE);
+	return kn;
+}
+
+static void snd_ctl_led_sysfs_exit(struct kernfs_node *kn)
+{
+	sysfs_unbreak_active_protection(kn);
+	module_put(THIS_MODULE);
+}
+
 static ssize_t attach_store(struct device *dev,
 			    struct device_attribute *attr,
 			    const char *buf, size_t count)
 {
 	struct snd_ctl_led_card *led_card = container_of(dev, struct snd_ctl_led_card, dev);
-	return set_led_id(led_card, buf, count, true);
+	struct kernfs_node *kn;
+	ssize_t ret;
+
+	kn = snd_ctl_led_sysfs_enter(dev, attr);
+	if (!kn)
+		return -ENODEV;
+	ret = set_led_id(led_card, buf, count, true);
+	snd_ctl_led_sysfs_exit(kn);
+	return ret;
 }
 
 static ssize_t detach_store(struct device *dev,
@@ -576,7 +607,15 @@ static ssize_t detach_store(struct device *dev,
 			    const char *buf, size_t count)
 {
 	struct snd_ctl_led_card *led_card = container_of(dev, struct snd_ctl_led_card, dev);
-	return set_led_id(led_card, buf, count, false);
+	struct kernfs_node *kn;
+	ssize_t ret;
+
+	kn = snd_ctl_led_sysfs_enter(dev, attr);
+	if (!kn)
+		return -ENODEV;
+	ret = set_led_id(led_card, buf, count, false);
+	snd_ctl_led_sysfs_exit(kn);
+	return ret;
 }
 
 static ssize_t reset_store(struct device *dev,
@@ -594,10 +633,8 @@ static ssize_t reset_store(struct device *dev,
 	return count;
 }
 
-static ssize_t list_show(struct device *dev,
-			 struct device_attribute *attr, char *buf)
+static ssize_t snd_ctl_led_list(struct snd_ctl_led_card *led_card, char *buf)
 {
-	struct snd_ctl_led_card *led_card = container_of(dev, struct snd_ctl_led_card, dev);
 	struct snd_ctl_led_ctl *lctl;
 	size_t l = 0;
 	struct snd_card *card __free(snd_card_unref) =
@@ -620,6 +657,21 @@ static ssize_t list_show(struct device *dev,
 	return l;
 }
 
+static ssize_t list_show(struct device *dev,
+			 struct device_attribute *attr, char *buf)
+{
+	struct snd_ctl_led_card *led_card = container_of(dev, struct snd_ctl_led_card, dev);
+	struct kernfs_node *kn;
+	ssize_t ret;
+
+	kn = snd_ctl_led_sysfs_enter(dev, attr);
+	if (!kn)
+		return -ENODEV;
+	ret = snd_ctl_led_list(led_card, buf);
+	snd_ctl_led_sysfs_exit(kn);
+	return ret;
+}
+
 static DEVICE_ATTR_WO(attach);
 static DEVICE_ATTR_WO(detach);
 static DEVICE_ATTR_WO(reset);
-- 
2.55.0

Re: [PATCH] ALSA: control_led: Fix deadlock between sysfs writes and card disconnect

[Takashi Iwai]

On Sun, 11 Oct 2026 20:06:17 +0200,
Ali Ahmet Memiş wrote:
> 
> snd_ctl_led_disconnect() removes the ctl-led sysfs files while
> holding card->controls_rwsem.

Hm, it's not clear to me how this happens: I see no controls_rwsem
down around snd_ctl_led_disconnect() call.  Could you elaborate about
it a bit more?


thanks,

Takashi

> device_unregister() waits for running
> sysfs callbacks which can deadlock when a concurrent write causes
> the attach or detach callback to wait for controls_rwsem.
> 
> Break kernfs active protection in the attach, detach and list
> callbacks and hold a module reference instead.
> 
> Fixes: a135dfb5de15 ("ALSA: led control - add sysfs kcontrol LED marking layer")
> Signed-off-by: Ali Ahmet Memiş <aliamemis@disroot.org>
> ---
>  sound/core/control_led.c | 62 ++++++++++++++++++++++++++++++++++++----
>  1 file changed, 57 insertions(+), 5 deletions(-)
> 
> diff --git a/sound/core/control_led.c b/sound/core/control_led.c
> index ec0e6c7ad..501387287 100644
> --- a/sound/core/control_led.c
> +++ b/sound/core/control_led.c
> @@ -563,12 +563,43 @@ static ssize_t set_led_id(struct snd_ctl_led_card *led_card, const char *buf, si
>  	return count;
>  }
>  
> +/*
> + * snd_ctl_led_disconnect() removes these files with card->controls_rwsem
> + * held. so don't let the removal wait for a callback that takes it.
> + */
> +static struct kernfs_node *snd_ctl_led_sysfs_enter(struct device *dev,
> +						   struct device_attribute *attr)
> +{
> +	struct kernfs_node *kn;
> +
> +	if (!try_module_get(THIS_MODULE))
> +		return NULL;
> +	kn = sysfs_break_active_protection(&dev->kobj, &attr->attr);
> +	if (!kn)
> +		module_put(THIS_MODULE);
> +	return kn;
> +}
> +
> +static void snd_ctl_led_sysfs_exit(struct kernfs_node *kn)
> +{
> +	sysfs_unbreak_active_protection(kn);
> +	module_put(THIS_MODULE);
> +}
> +
>  static ssize_t attach_store(struct device *dev,
>  			    struct device_attribute *attr,
>  			    const char *buf, size_t count)
>  {
>  	struct snd_ctl_led_card *led_card = container_of(dev, struct snd_ctl_led_card, dev);
> -	return set_led_id(led_card, buf, count, true);
> +	struct kernfs_node *kn;
> +	ssize_t ret;
> +
> +	kn = snd_ctl_led_sysfs_enter(dev, attr);
> +	if (!kn)
> +		return -ENODEV;
> +	ret = set_led_id(led_card, buf, count, true);
> +	snd_ctl_led_sysfs_exit(kn);
> +	return ret;
>  }
>  
>  static ssize_t detach_store(struct device *dev,
> @@ -576,7 +607,15 @@ static ssize_t detach_store(struct device *dev,
>  			    const char *buf, size_t count)
>  {
>  	struct snd_ctl_led_card *led_card = container_of(dev, struct snd_ctl_led_card, dev);
> -	return set_led_id(led_card, buf, count, false);
> +	struct kernfs_node *kn;
> +	ssize_t ret;
> +
> +	kn = snd_ctl_led_sysfs_enter(dev, attr);
> +	if (!kn)
> +		return -ENODEV;
> +	ret = set_led_id(led_card, buf, count, false);
> +	snd_ctl_led_sysfs_exit(kn);
> +	return ret;
>  }
>  
>  static ssize_t reset_store(struct device *dev,
> @@ -594,10 +633,8 @@ static ssize_t reset_store(struct device *dev,
>  	return count;
>  }
>  
> -static ssize_t list_show(struct device *dev,
> -			 struct device_attribute *attr, char *buf)
> +static ssize_t snd_ctl_led_list(struct snd_ctl_led_card *led_card, char *buf)
>  {
> -	struct snd_ctl_led_card *led_card = container_of(dev, struct snd_ctl_led_card, dev);
>  	struct snd_ctl_led_ctl *lctl;
>  	size_t l = 0;
>  	struct snd_card *card __free(snd_card_unref) =
> @@ -620,6 +657,21 @@ static ssize_t list_show(struct device *dev,
>  	return l;
>  }
>  
> +static ssize_t list_show(struct device *dev,
> +			 struct device_attribute *attr, char *buf)
> +{
> +	struct snd_ctl_led_card *led_card = container_of(dev, struct snd_ctl_led_card, dev);
> +	struct kernfs_node *kn;
> +	ssize_t ret;
> +
> +	kn = snd_ctl_led_sysfs_enter(dev, attr);
> +	if (!kn)
> +		return -ENODEV;
> +	ret = snd_ctl_led_list(led_card, buf);
> +	snd_ctl_led_sysfs_exit(kn);
> +	return ret;
> +}
> +
>  static DEVICE_ATTR_WO(attach);
>  static DEVICE_ATTR_WO(detach);
>  static DEVICE_ATTR_WO(reset);
> -- 
> 2.55.0
> 

Re: [PATCH] ALSA: control_led: Fix deadlock between sysfs writes and card disconnect

[Ali Ahmet Memiş]

On Mon, 12 Oct 2026 09:24:26 +0200, Takashi Iwai wrote:
> Hm, it's not clear to me how this happens: I see no controls_rwsem
> down around snd_ctl_led_disconnect() call.  Could you elaborate about
> it a bit more?

It's taken in the call_snd_ctl_lops() macro in control.c:

	guard(rwsem_read)(&(_card)->controls_rwsem);
	guard(rwsem_read)(&snd_ctl_layer_rwsem);

so snd_ctl_dev_disconnect() calls ->ldisconnect() with controls_rwsem
held for read.  snd_ctl_led_sysfs_remove() then waits in kernfs_drain()
for the attach write which is blocked on down_write(controls_rwsem) in
snd_ctl_led_set_id().

I hit it in QEMU with snd-dummy writing to attach in a loop while
unloading the module.  It hung on the third iteration:

rmmod:
[<0>] kernfs_drain+0x2eb/0x510
[<0>] __kernfs_remove+0x2e6/0x890
[<0>] kernfs_remove_by_name_ns+0x7d/0x100
[<0>] remove_files+0x8d/0x1b0
[<0>] sysfs_remove_group+0x71/0xd0
[<0>] sysfs_remove_groups+0x67/0xd0
[<0>] device_remove_attrs+0x181/0x270
[<0>] device_del+0x31f/0x8d0
[<0>] device_unregister+0x12/0x30
[<0>] snd_ctl_led_sysfs_remove+0x161/0x2f0
[<0>] snd_ctl_led_disconnect+0x16/0xd0
[<0>] snd_ctl_dev_disconnect+0x151/0x210
[<0>] snd_device_disconnect_all+0xf0/0x180
[<0>] snd_card_disconnect.part.0+0x25d/0x4c0

attach writer:
[<0>] set_led_id+0x2d9/0xc60
[<0>] kernfs_fop_write_iter+0x33c/0x510
[<0>] vfs_write+0x516/0x1340

and lockdep reports it as well:

  Possible unsafe locking scenario:
        CPU0                    CPU1
        ----                    ----
   rlock(snd_ctl_layer_rwsem);
                                lock(&card->controls_rwsem);
                                lock(snd_ctl_layer_rwsem);
   lock(kn->active#37);

   #1: ffff88800768a1f0 (&card->controls_rwsem){++++}-{4:4}, at: snd_ctl_dev_disconnect+0x10e/0x210
   #2: ffffffff881fdf28 (snd_ctl_layer_rwsem){++++}-{4:4}, at: snd_ctl_dev_disconnect+0x11a/0x210

Re: [PATCH] ALSA: control_led: Fix deadlock between sysfs writes and card disconnect

[Takashi Iwai]

On Mon, 12 Oct 2026 16:12:35 +0200,
Ali Ahmet Memiş wrote:
> 
> On Mon, 12 Oct 2026 09:24:26 +0200, Takashi Iwai wrote:
> > Hm, it's not clear to me how this happens: I see no controls_rwsem
> > down around snd_ctl_led_disconnect() call.  Could you elaborate about
> > it a bit more?
> 
> It's taken in the call_snd_ctl_lops() macro in control.c:
> 
> 	guard(rwsem_read)(&(_card)->controls_rwsem);
> 	guard(rwsem_read)(&snd_ctl_layer_rwsem);
> 
> so snd_ctl_dev_disconnect() calls ->ldisconnect() with controls_rwsem
> held for read.  snd_ctl_led_sysfs_remove() then waits in kernfs_drain()
> for the attach write which is blocked on down_write(controls_rwsem) in
> snd_ctl_led_set_id().

Thanks, now it's clearer.
The patch is applied to for-next branch now.


thanks,

Takashi