ALSA: control: Optimize locking for look-up

For a fast look-up of a control element via either numid or name
matching (enabled via CONFIG_SND_CTL_FAST_LOOKUP), a locking isn't
needed at all thanks to Xarray.  OTOH, the locking is still needed for
a slow linked-list traversal, and that's rather a rare case.

In this patch, we reduce the use of locking at snd_ctl_find_*() API
functions, and switch from controls_rwsem to controls_rwlock for
avoiding unnecessary lock inversions.  This also resulted in a nice
cleanup, as *_unlocked() version of snd_ctl_find_*() APIs can be
dropped.

snd_ctl_find_id_mixer_unlocked() is still left just as an alias of
snd_ctl_find_id_mixer(), since soc-card.c has a wrapper and there are
several users.  Once after converting there, we can remove it later.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20240809104234.8488-3-tiwai@suse.de

include/sound/control.h

@@ -140,9 +140,7 @@ int snd_ctl_remove_id(struct snd_card * card, struct snd_ctl_elem_id *id);
 int snd_ctl_rename_id(struct snd_card * card, struct snd_ctl_elem_id *src_id, struct snd_ctl_elem_id *dst_id);
 void snd_ctl_rename(struct snd_card *card, struct snd_kcontrol *kctl, const char *name);
 int snd_ctl_activate_id(struct snd_card *card, struct snd_ctl_elem_id *id, int active);
-struct snd_kcontrol *snd_ctl_find_numid_locked(struct snd_card *card, unsigned int numid);
 struct snd_kcontrol *snd_ctl_find_numid(struct snd_card *card, unsigned int numid);
-struct snd_kcontrol *snd_ctl_find_id_locked(struct snd_card *card, const struct snd_ctl_elem_id *id);
 struct snd_kcontrol *snd_ctl_find_id(struct snd_card *card, const struct snd_ctl_elem_id *id);
 
 /**
@@ -167,27 +165,11 @@ snd_ctl_find_id_mixer(struct snd_card *card, const char *name)
 	return snd_ctl_find_id(card, &id);
 }
 
-/**
- * snd_ctl_find_id_mixer_locked - find the control instance with the given name string
- * @card: the card instance
- * @name: the name string
- *
- * Finds the control instance with the given name and
- * @SNDRV_CTL_ELEM_IFACE_MIXER. Other fields are set to zero.
- *
- * This is merely a wrapper to snd_ctl_find_id_locked().
- * The caller must down card->controls_rwsem before calling this function.
- *
- * Return: The pointer of the instance if found, or %NULL if not.
- */
+/* to be removed */
 static inline struct snd_kcontrol *
 snd_ctl_find_id_mixer_locked(struct snd_card *card, const char *name)
 {
-	struct snd_ctl_elem_id id = {};
-
-	id.iface = SNDRV_CTL_ELEM_IFACE_MIXER;
-	strscpy(id.name, name, sizeof(id.name));
-	return snd_ctl_find_id_locked(card, &id);
+	return snd_ctl_find_id_mixer(card, name);
 }
 
 int snd_ctl_create(struct snd_card *card);

include/sound/core.h

@@ -99,7 +99,7 @@ struct snd_card {
 	struct device *ctl_dev;		/* control device */
 	unsigned int last_numid;	/* last used numeric ID */
 	struct rw_semaphore controls_rwsem;	/* controls lock (list and values) */
-	rwlock_t controls_rwlock;	/* lock for ctl_files list */
+	rwlock_t controls_rwlock;	/* lock for lookup and ctl_files list */
 	int controls_count;		/* count of all controls */
 	size_t user_ctl_alloc_size;	// current memory allocation by user controls.
 	struct list_head controls;	/* all controls for this card */

sound/core/control.c

@@ -470,7 +470,7 @@ static int __snd_ctl_add_replace(struct snd_card *card,
 	if (id.index > UINT_MAX - kcontrol->count)
 		return -EINVAL;
 
-	old = snd_ctl_find_id_locked(card, &id);
+	old = snd_ctl_find_id(card, &id);
 	if (!old) {
 		if (mode == CTL_REPLACE)
 			return -EINVAL;
@@ -491,10 +491,12 @@ static int __snd_ctl_add_replace(struct snd_card *card,
 	if (snd_ctl_find_hole(card, kcontrol->count) < 0)
 		return -ENOMEM;
 
-	list_add_tail(&kcontrol->list, &card->controls);
-	card->controls_count += kcontrol->count;
-	kcontrol->id.numid = card->last_numid + 1;
-	card->last_numid += kcontrol->count;
+	scoped_guard(write_lock_irq, &card->controls_rwlock) {
+		list_add_tail(&kcontrol->list, &card->controls);
+		card->controls_count += kcontrol->count;
+		kcontrol->id.numid = card->last_numid + 1;
+		card->last_numid += kcontrol->count;
+	}
 
 	add_hash_entries(card, kcontrol);
 
@@ -579,12 +581,15 @@ static int __snd_ctl_remove(struct snd_card *card,
 
 	if (snd_BUG_ON(!card || !kcontrol))
 		return -EINVAL;
-	list_del(&kcontrol->list);
 
 	if (remove_hash)
 		remove_hash_entries(card, kcontrol);
 
-	card->controls_count -= kcontrol->count;
+	scoped_guard(write_lock_irq, &card->controls_rwlock) {
+		list_del(&kcontrol->list);
+		card->controls_count -= kcontrol->count;
+	}
+
 	for (idx = 0; idx < kcontrol->count; idx++)
 		snd_ctl_notify_one(card, SNDRV_CTL_EVENT_MASK_REMOVE, kcontrol, idx);
 	snd_ctl_free_one(kcontrol);
@@ -634,7 +639,7 @@ int snd_ctl_remove_id(struct snd_card *card, struct snd_ctl_elem_id *id)
 	struct snd_kcontrol *kctl;
 
 	guard(rwsem_write)(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, id);
+	kctl = snd_ctl_find_id(card, id);
 	if (kctl == NULL)
 		return -ENOENT;
 	return snd_ctl_remove_locked(card, kctl);
@@ -659,7 +664,7 @@ static int snd_ctl_remove_user_ctl(struct snd_ctl_file * file,
 	int idx;
 
 	guard(rwsem_write)(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, id);
+	kctl = snd_ctl_find_id(card, id);
 	if (kctl == NULL)
 		return -ENOENT;
 	if (!(kctl->vd[0].access & SNDRV_CTL_ELEM_ACCESS_USER))
@@ -691,7 +696,7 @@ int snd_ctl_activate_id(struct snd_card *card, struct snd_ctl_elem_id *id,
 	int ret;
 
 	down_write(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, id);
+	kctl = snd_ctl_find_id(card, id);
 	if (kctl == NULL) {
 		ret = -ENOENT;
 		goto unlock;
@@ -745,7 +750,7 @@ int snd_ctl_rename_id(struct snd_card *card, struct snd_ctl_elem_id *src_id,
 	int saved_numid;
 
 	guard(rwsem_write)(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, src_id);
+	kctl = snd_ctl_find_id(card, src_id);
 	if (kctl == NULL)
 		return -ENOENT;
 	saved_numid = kctl->id.numid;
@@ -787,6 +792,7 @@ snd_ctl_find_numid_slow(struct snd_card *card, unsigned int numid)
 {
 	struct snd_kcontrol *kctl;
 
+	guard(read_lock_irqsave)(&card->controls_rwlock);
 	list_for_each_entry(kctl, &card->controls, list) {
 		if (kctl->id.numid <= numid && kctl->id.numid + kctl->count > numid)
 			return kctl;
@@ -796,72 +802,51 @@ snd_ctl_find_numid_slow(struct snd_card *card, unsigned int numid)
 #endif /* !CONFIG_SND_CTL_FAST_LOOKUP */
 
 /**
- * snd_ctl_find_numid_locked - find the control instance with the given number-id
+ * snd_ctl_find_numid - find the control instance with the given number-id
  * @card: the card instance
  * @numid: the number-id to search
  *
  * Finds the control instance with the given number-id from the card.
  *
- * The caller must down card->controls_rwsem before calling this function
- * (if the race condition can happen).
- *
  * Return: The pointer of the instance if found, or %NULL if not.
+ *
+ * Note that this function takes card->controls_rwlock lock internally.
  */
-struct snd_kcontrol *
-snd_ctl_find_numid_locked(struct snd_card *card, unsigned int numid)
+struct snd_kcontrol *snd_ctl_find_numid(struct snd_card *card,
+					unsigned int numid)
 {
 	if (snd_BUG_ON(!card || !numid))
 		return NULL;
-	lockdep_assert_held(&card->controls_rwsem);
+
 #ifdef CONFIG_SND_CTL_FAST_LOOKUP
 	return xa_load(&card->ctl_numids, numid);
 #else
 	return snd_ctl_find_numid_slow(card, numid);
 #endif
 }
-EXPORT_SYMBOL(snd_ctl_find_numid_locked);
-
-/**
- * snd_ctl_find_numid - find the control instance with the given number-id
- * @card: the card instance
- * @numid: the number-id to search
- *
- * Finds the control instance with the given number-id from the card.
- *
- * Return: The pointer of the instance if found, or %NULL if not.
- *
- * Note that this function takes card->controls_rwsem lock internally.
- */
-struct snd_kcontrol *snd_ctl_find_numid(struct snd_card *card,
-					unsigned int numid)
-{
-	guard(rwsem_read)(&card->controls_rwsem);
-	return snd_ctl_find_numid_locked(card, numid);
-}
 EXPORT_SYMBOL(snd_ctl_find_numid);
 
 /**
- * snd_ctl_find_id_locked - find the control instance with the given id
+ * snd_ctl_find_id - find the control instance with the given id
  * @card: the card instance
  * @id: the id to search
  *
  * Finds the control instance with the given id from the card.
  *
- * The caller must down card->controls_rwsem before calling this function
- * (if the race condition can happen).
- *
  * Return: The pointer of the instance if found, or %NULL if not.
+ *
+ * Note that this function takes card->controls_rwlock lock internally.
  */
-struct snd_kcontrol *snd_ctl_find_id_locked(struct snd_card *card,
-					    const struct snd_ctl_elem_id *id)
+struct snd_kcontrol *snd_ctl_find_id(struct snd_card *card,
+				     const struct snd_ctl_elem_id *id)
 {
 	struct snd_kcontrol *kctl;
 
 	if (snd_BUG_ON(!card || !id))
 		return NULL;
-	lockdep_assert_held(&card->controls_rwsem);
+
 	if (id->numid != 0)
-		return snd_ctl_find_numid_locked(card, id->numid);
+		return snd_ctl_find_numid(card, id->numid);
 #ifdef CONFIG_SND_CTL_FAST_LOOKUP
 	kctl = xa_load(&card->ctl_hash, get_ctl_id_hash(id));
 	if (kctl && elem_id_matches(kctl, id))
@@ -870,31 +855,13 @@ struct snd_kcontrol *snd_ctl_find_id_locked(struct snd_card *card,
 		return NULL; /* we can rely on only hash table */
 #endif
 	/* no matching in hash table - try all as the last resort */
+	guard(read_lock_irqsave)(&card->controls_rwlock);
 	list_for_each_entry(kctl, &card->controls, list)
 		if (elem_id_matches(kctl, id))
 			return kctl;
 
 	return NULL;
 }
-EXPORT_SYMBOL(snd_ctl_find_id_locked);
-
-/**
- * snd_ctl_find_id - find the control instance with the given id
- * @card: the card instance
- * @id: the id to search
- *
- * Finds the control instance with the given id from the card.
- *
- * Return: The pointer of the instance if found, or %NULL if not.
- *
- * Note that this function takes card->controls_rwsem lock internally.
- */
-struct snd_kcontrol *snd_ctl_find_id(struct snd_card *card,
-				     const struct snd_ctl_elem_id *id)
-{
-	guard(rwsem_read)(&card->controls_rwsem);
-	return snd_ctl_find_id_locked(card, id);
-}
 EXPORT_SYMBOL(snd_ctl_find_id);
 
 static int snd_ctl_card_info(struct snd_card *card, struct snd_ctl_file * ctl,
@@ -1199,7 +1166,7 @@ static int snd_ctl_elem_info(struct snd_ctl_file *ctl,
 	struct snd_kcontrol *kctl;
 
 	guard(rwsem_read)(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, &info->id);
+	kctl = snd_ctl_find_id(card, &info->id);
 	if (!kctl)
 		return -ENOENT;
 	return __snd_ctl_elem_info(card, kctl, info, ctl);
@@ -1235,7 +1202,7 @@ static int snd_ctl_elem_read(struct snd_card *card,
 	int ret;
 
 	guard(rwsem_read)(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, &control->id);
+	kctl = snd_ctl_find_id(card, &control->id);
 	if (!kctl)
 		return -ENOENT;
 
@@ -1303,7 +1270,7 @@ static int snd_ctl_elem_write(struct snd_card *card, struct snd_ctl_file *file,
 	int result;
 
 	down_write(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, &control->id);
+	kctl = snd_ctl_find_id(card, &control->id);
 	if (kctl == NULL) {
 		up_write(&card->controls_rwsem);
 		return -ENOENT;
@@ -1381,7 +1348,7 @@ static int snd_ctl_elem_lock(struct snd_ctl_file *file,
 	if (copy_from_user(&id, _id, sizeof(id)))
 		return -EFAULT;
 	guard(rwsem_write)(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, &id);
+	kctl = snd_ctl_find_id(card, &id);
 	if (!kctl)
 		return -ENOENT;
 	vd = &kctl->vd[snd_ctl_get_ioff(kctl, &id)];
@@ -1402,7 +1369,7 @@ static int snd_ctl_elem_unlock(struct snd_ctl_file *file,
 	if (copy_from_user(&id, _id, sizeof(id)))
 		return -EFAULT;
 	guard(rwsem_write)(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, &id);
+	kctl = snd_ctl_find_id(card, &id);
 	if (!kctl)
 		return -ENOENT;
 	vd = &kctl->vd[snd_ctl_get_ioff(kctl, &id)];
@@ -1903,7 +1870,7 @@ static int snd_ctl_tlv_ioctl(struct snd_ctl_file *file,
 	container_size = header.length;
 	container = buf->tlv;
 
-	kctl = snd_ctl_find_numid_locked(file->card, header.numid);
+	kctl = snd_ctl_find_numid(file->card, header.numid);
 	if (kctl == NULL)
 		return -ENOENT;
 

sound/core/control_compat.c

@@ -168,7 +168,7 @@ static int get_ctl_type(struct snd_card *card, struct snd_ctl_elem_id *id,
 	int err;
 
 	guard(rwsem_read)(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, id);
+	kctl = snd_ctl_find_id(card, id);
 	if (!kctl)
 		return -ENOENT;
 	info = kzalloc(sizeof(*info), GFP_KERNEL);

sound/core/control_led.c

@@ -254,7 +254,7 @@ static int snd_ctl_led_set_id(int card_number, struct snd_ctl_elem_id *id,
 	if (!card)
 		return -ENXIO;
 	guard(rwsem_write)(&card->controls_rwsem);
-	kctl = snd_ctl_find_id_locked(card, id);
+	kctl = snd_ctl_find_id(card, id);
 	if (!kctl)
 		return -ENOENT;
 	ioff = snd_ctl_get_ioff(kctl, id);

sound/core/oss/mixer_oss.c

@@ -510,7 +510,7 @@ static struct snd_kcontrol *snd_mixer_oss_test_id(struct snd_mixer_oss *mixer, c
 	id.iface = SNDRV_CTL_ELEM_IFACE_MIXER;
 	strscpy(id.name, name, sizeof(id.name));
 	id.index = index;
-	return snd_ctl_find_id_locked(card, &id);
+	return snd_ctl_find_id(card, &id);
 }
 
 static void snd_mixer_oss_get_volume1_vol(struct snd_mixer_oss_file *fmixer,
@@ -526,7 +526,7 @@ static void snd_mixer_oss_get_volume1_vol(struct snd_mixer_oss_file *fmixer,
 	if (numid == ID_UNKNOWN)
 		return;
 	guard(rwsem_read)(&card->controls_rwsem);
-	kctl = snd_ctl_find_numid_locked(card, numid);
+	kctl = snd_ctl_find_numid(card, numid);
 	if (!kctl)
 		return;
 	uinfo = kzalloc(sizeof(*uinfo), GFP_KERNEL);
@@ -559,7 +559,7 @@ static void snd_mixer_oss_get_volume1_sw(struct snd_mixer_oss_file *fmixer,
 	if (numid == ID_UNKNOWN)
 		return;
 	guard(rwsem_read)(&card->controls_rwsem);
-	kctl = snd_ctl_find_numid_locked(card, numid);
+	kctl = snd_ctl_find_numid(card, numid);
 	if (!kctl)
 		return;
 	uinfo = kzalloc(sizeof(*uinfo), GFP_KERNEL);
@@ -619,7 +619,7 @@ static void snd_mixer_oss_put_volume1_vol(struct snd_mixer_oss_file *fmixer,
 	if (numid == ID_UNKNOWN)
 		return;
 	guard(rwsem_read)(&card->controls_rwsem);
-	kctl = snd_ctl_find_numid_locked(card, numid);
+	kctl = snd_ctl_find_numid(card, numid);
 	if (!kctl)
 		return;
 	uinfo = kzalloc(sizeof(*uinfo), GFP_KERNEL);
@@ -656,7 +656,7 @@ static void snd_mixer_oss_put_volume1_sw(struct snd_mixer_oss_file *fmixer,
 	if (numid == ID_UNKNOWN)
 		return;
 	guard(rwsem_read)(&card->controls_rwsem);
-	kctl = snd_ctl_find_numid_locked(card, numid);
+	kctl = snd_ctl_find_numid(card, numid);
 	if (!kctl)
 		return;
 	uinfo = kzalloc(sizeof(*uinfo), GFP_KERNEL);