staging: greybus: Avoid abusing controls_rwsem

The controls_rwsem of snd_card object is rather an internal lock, and
not really meant to be used by others for its data protection.

This patch addresses it by replacing the controls_rwsem usages with
the own (new) mutex.

Note that the up_write() and down_write() calls around
gbaudio_remove_component_controls() are simply dropped without
replacement. These temporary up/down were a workaround since
gbaudio_remove_component_controls() itself took the rwsem. Now it was
also gone, we can clean up the workaround, too.

Acked-by: Greg Kroah-Hartman <[e-mail]>
Cc: Vaibhav Agarwal <[e-mail]>
Cc: Mark Greer <[e-mail]>
Cc: Johan Hovold <[e-mail]>
Cc: Alex Elder <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

drivers/staging/greybus/audio_codec.c

···
 {
 	int ret;
 	struct snd_soc_component *comp;
-	struct snd_card *card;
 	struct gbaudio_jack *jack = NULL;
 
 	if (!gbcodec) {
···
 	}
 
 	comp = gbcodec->component;
-	card = comp->card->snd_card;
 
-	down_write(&card->controls_rwsem);
+	mutex_lock(&gbcodec->register_mutex);
 
 	if (module->num_dais) {
 		dev_err(gbcodec->dev,
 			"%d:DAIs not supported via gbcodec driver\n",
 			module->num_dais);
-		up_write(&card->controls_rwsem);
+		mutex_unlock(&gbcodec->register_mutex);
 		return -EINVAL;
 	}
 
 	ret = gbaudio_init_jack(module, comp->card);
 	if (ret) {
-		up_write(&card->controls_rwsem);
+		mutex_unlock(&gbcodec->register_mutex);
 		return ret;
 	}
 
···
 		ret = snd_soc_dapm_new_widgets(comp->card);
 	dev_dbg(comp->dev, "Registered %s module\n", module->name);
 
-	up_write(&card->controls_rwsem);
+	mutex_unlock(&gbcodec->register_mutex);
 	return ret;
 }
 EXPORT_SYMBOL(gbaudio_register_module);
···
 void gbaudio_unregister_module(struct gbaudio_module_info *module)
 {
 	struct snd_soc_component *comp = gbcodec->component;
-	struct snd_card *card = comp->card->snd_card;
 	struct gbaudio_jack *jack, *n;
 	int mask;
 
 	dev_dbg(comp->dev, "Unregister %s module\n", module->name);
 
-	down_write(&card->controls_rwsem);
+	mutex_lock(&gbcodec->register_mutex);
 	mutex_lock(&gbcodec->lock);
 	gbaudio_codec_cleanup(module);
 	list_del(&module->list);
···
 		dev_dbg(comp->dev, "Removing %d controls\n",
 			module->num_controls);
 		/* release control semaphore */
-		up_write(&card->controls_rwsem);
 		gbaudio_remove_component_controls(comp, module->controls,
 						  module->num_controls);
-		down_write(&card->controls_rwsem);
 	}
 	if (module->dapm_widgets) {
 		dev_dbg(comp->dev, "Removing %d widgets\n",
···
 
 	dev_dbg(comp->dev, "Unregistered %s module\n", module->name);
 
-	up_write(&card->controls_rwsem);
+	mutex_unlock(&gbcodec->register_mutex);
 }
 EXPORT_SYMBOL(gbaudio_unregister_module);
 
···
 	info->dev = comp->dev;
 	INIT_LIST_HEAD(&info->module_list);
 	mutex_init(&info->lock);
+	mutex_init(&info->register_mutex);
 	INIT_LIST_HEAD(&info->dai_list);
 
 	/* init dai_list used to maintain runtime stream info */

drivers/staging/greybus/audio_codec.h

···
 	/* to maintain runtime stream params for each DAI */
 	struct list_head dai_list;
 	struct mutex lock;
+	struct mutex register_mutex;
 };
 
 struct gbaudio_widget {