ALSA: emux: Fix mutex deadlock in OSS emulation

commit 1c94e65c upstream.

The OSS emulation in synth-emux helper has a potential AB/BA deadlock
at the simultaneous closing and opening:

  close ->
    snd_seq_release() ->
      sne_seq_free_client() ->
        snd_seq_delete_all_ports(): takes client->ports_mutex ->
	  port_delete() ->
	    snd_emux_unuse(): takes emux->register_mutex

  open ->
    snd_seq_oss_open() ->
      snd_emux_open_seq_oss(): takes emux->register_mutex ->
        snd_seq_event_port_attach() ->
	  snd_seq_create_port(): takes client->ports_mutex

This patch addresses the deadlock by reducing the rance taking
emux->register_mutex in snd_emux_open_seq_oss().  The lock is needed
for the refcount handling, so move it locally.  The calls in
emux_seq.c are already with the mutex, thus they are replaced with the
version without mutex lock/unlock.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

sound/synth/emux/emux_oss.c

@@ -118,12 +118,8 @@ snd_emux_open_seq_oss(struct snd_seq_oss_arg *arg, void *closure)
 	if (snd_BUG_ON(!arg || !emu))
 		return -ENXIO;
 
-	mutex_lock(&emu->register_mutex);
-
-	if (!snd_emux_inc_count(emu)) {
-		mutex_unlock(&emu->register_mutex);
+	if (!snd_emux_inc_count(emu))
 		return -EFAULT;
-	}
 
 	memset(&callback, 0, sizeof(callback));
 	callback.owner = THIS_MODULE;
@@ -135,7 +131,6 @@ snd_emux_open_seq_oss(struct snd_seq_oss_arg *arg, void *closure)
 	if (p == NULL) {
 		snd_printk(KERN_ERR "can't create port\n");
 		snd_emux_dec_count(emu);
-		mutex_unlock(&emu->register_mutex);
 		return -ENOMEM;
 	}
 
@@ -148,8 +143,6 @@ snd_emux_open_seq_oss(struct snd_seq_oss_arg *arg, void *closure)
 	reset_port_mode(p, arg->seq_mode);
 
 	snd_emux_reset_port(p);
-
-	mutex_unlock(&emu->register_mutex);
 	return 0;
 }
 
@@ -195,13 +188,11 @@ snd_emux_close_seq_oss(struct snd_seq_oss_arg *arg)
 	if (snd_BUG_ON(!emu))
 		return -ENXIO;
 
-	mutex_lock(&emu->register_mutex);
 	snd_emux_sounds_off_all(p);
 	snd_soundfont_close_check(emu->sflist, SF_CLIENT_NO(p->chset.port));
 	snd_seq_event_port_detach(p->chset.client, p->chset.port);
 	snd_emux_dec_count(emu);
 
-	mutex_unlock(&emu->register_mutex);
 	return 0;
 }
 

sound/synth/emux/emux_seq.c

@@ -267,8 +267,8 @@ snd_emux_event_input(struct snd_seq_event *ev, int direct, void *private_data,
 /*
  * increment usage count
  */
-int
-snd_emux_inc_count(struct snd_emux *emu)
+static int
+__snd_emux_inc_count(struct snd_emux *emu)
 {
 	emu->used++;
 	if (!try_module_get(emu->ops.owner))
@@ -282,12 +282,21 @@ snd_emux_inc_count(struct snd_emux *emu)
 	return 1;
 }
 
+int snd_emux_inc_count(struct snd_emux *emu)
+{
+	int ret;
+
+	mutex_lock(&emu->register_mutex);
+	ret = __snd_emux_inc_count(emu);
+	mutex_unlock(&emu->register_mutex);
+	return ret;
+}
 
 /*
  * decrease usage count
  */
-void
-snd_emux_dec_count(struct snd_emux *emu)
+static void
+__snd_emux_dec_count(struct snd_emux *emu)
 {
 	module_put(emu->card->module);
 	emu->used--;
@@ -296,6 +305,12 @@ snd_emux_dec_count(struct snd_emux *emu)
 	module_put(emu->ops.owner);
 }
 
+void snd_emux_dec_count(struct snd_emux *emu)
+{
+	mutex_lock(&emu->register_mutex);
+	__snd_emux_dec_count(emu);
+	mutex_unlock(&emu->register_mutex);
+}
 
 /*
  * Routine that is called upon a first use of a particular port
@@ -315,7 +330,7 @@ snd_emux_use(void *private_data, struct snd_seq_port_subscribe *info)
 
 	mutex_lock(&emu->register_mutex);
 	snd_emux_init_port(p);
-	snd_emux_inc_count(emu);
+	__snd_emux_inc_count(emu);
 	mutex_unlock(&emu->register_mutex);
 	return 0;
 }
@@ -338,7 +353,7 @@ snd_emux_unuse(void *private_data, struct snd_seq_port_subscribe *info)
 
 	mutex_lock(&emu->register_mutex);
 	snd_emux_sounds_off_all(p);
-	snd_emux_dec_count(emu);
+	__snd_emux_dec_count(emu);
 	mutex_unlock(&emu->register_mutex);
 	return 0;
 }