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>
diff --git a/sound/synth/emux/emux_seq.c b/sound/synth/emux/emux_seq.c
index 188fda0e..a020920 100644
--- a/sound/synth/emux/emux_seq.c
+++ b/sound/synth/emux/emux_seq.c
@@ -267,8 +267,8 @@
/*
* 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 @@
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 @@
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 @@
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 @@
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;
}