Patch "ALSA: timer: Use mod_timer() for rearming the system timer" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

ALSA: timer: Use mod_timer() for rearming the system timer

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
alsa-timer-use-mod_timer-for-rearming-the-system-timer.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Takashi Iwai <[email protected]>
Date: Fri, 1 Apr 2016 12:28:16 +0200
Subject: ALSA: timer: Use mod_timer() for rearming the system timer

From: Takashi Iwai <[email protected]>

commit 4a07083ed613644c96c34a7dd2853dc5d7c70902 upstream.

ALSA system timer backend stops the timer via del_timer() without sync and leaves del_timer_sync() at the close instead. This is because of the restriction by the design of ALSA timer: namely, the stop callback may be called from the timer handler, and calling the sync shall lead to a hangup. However, this also triggers a kernel BUG() when the timer is rearmed immediately after stopping without sync:
kernel BUG at kernel/time/timer.c:966!

 Call Trace:
  <IRQ>
  [<ffffffff8239c94e>] snd_timer_s_start+0x13e/0x1a0
  [<ffffffff8239e1f4>] snd_timer_interrupt+0x504/0xec0
  [<ffffffff8122fca0>] ? debug_check_no_locks_freed+0x290/0x290
  [<ffffffff8239ec64>] snd_timer_s_function+0xb4/0x120
  [<ffffffff81296b72>] call_timer_fn+0x162/0x520
  [<ffffffff81296add>] ? call_timer_fn+0xcd/0x520
  [<ffffffff8239ebb0>] ? snd_timer_interrupt+0xec0/0xec0
  ....

It's the place where add_timer() checks the pending timer. It's clear that this may happen after the immediate restart without sync in our cases.

So, the workaround here is just to use mod_timer() instead of add_timer(). This looks like a band-aid fix, but it's a right move, as snd_timer_interrupt() takes care of the continuous rearm of timer.

---
 sound/core/timer.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -1012,8 +1012,8 @@ static int snd_timer_s_start(struct snd_
 		njiff += timer->sticks - priv->correction;
 		priv->correction = 0;
 	}
-	priv->last_expires = priv->tlist.expires = njiff;
-	add_timer(&priv->tlist);
+	priv->last_expires = njiff;
+	mod_timer(&priv->tlist, njiff);
 	return 0;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/alsa-timer-use-mod_timer-for-rearming-the-system-timer.patch