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Subject[PATCH 4.0 31/72] ALSA: emux: Fix mutex deadlock in OSS emulation
Date
4.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@suse.de>

commit 1c94e65c668f44d2c69ae7e7fc268ab3268fba3e 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
sound/synth/emux/emux_oss.c | 11 +----------
sound/synth/emux/emux_seq.c | 27 +++++++++++++++++++++------
2 files changed, 22 insertions(+), 16 deletions(-)

--- 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
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
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
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_os
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;
}

--- 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_even
/*
* 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

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, struc

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;
}



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