[PATCH] ALSA: Use del_timer_sync() before freeing timer

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>

The current code for freeing the emux timer is extremely dangerous:

  CPU0				CPU1
  ----				----
snd_emux_timer_callback()
			    snd_emux_free()
			      spin_lock(&emu->voice_lock)
			      del_timer(&emu->tlist); <-- returns immediately
			      spin_unlock(&emu->voice_lock);
			      [..]
			      kfree(emu);

  spin_lock(&emu->voice_lock);

 [BOOM!]

Instead just use del_timer_sync() which will wait for the timer to finish
before continuing. No need to check if the timer is active or not when
doing so.

This doesn't fix the race of a possible re-arming of the timer, but at
least it won't use the data that has just been freed.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
 sound/synth/emux/emux.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/sound/synth/emux/emux.c b/sound/synth/emux/emux.c
index 5ed8e36d2e04..a2ee78809cfb 100644
--- a/sound/synth/emux/emux.c
+++ b/sound/synth/emux/emux.c
@@ -131,10 +131,7 @@ int snd_emux_free(struct snd_emux *emu)
 	if (! emu)
 		return -EINVAL;
 
-	spin_lock_irqsave(&emu->voice_lock, flags);
-	if (emu->timer_active)
-		del_timer(&emu->tlist);
-	spin_unlock_irqrestore(&emu->voice_lock, flags);
+	del_timer_sync(&emu->tlist);
 
 	snd_emux_proc_free(emu);
 	snd_emux_delete_virmidi(emu);
-- 
2.35.1