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>

1 files changed, 1 insertions, 10 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;
 }