[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@xxxxxxxxxxx>
---
 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