ALSA: emux: Fix mutex deadlock in OSS emulation

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.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

2 files changed, 22 insertions, 16 deletions

diff --git a/sound/synth/emux/emux_oss.c b/sound/synth/emux/emux_oss.c
index ab37add269ae..82e350e9501c 100644
--- a/sound/synth/emux/emux_oss.c
+++ b/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;
 }
 
diff --git a/sound/synth/emux/emux_seq.c b/sound/synth/emux/emux_seq.c
index 188fda0effb0..a0209204ae48 100644
--- a/sound/synth/emux/emux_seq.c
+++ b/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;
 }