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From: Ben Hutchings <ben@decadent.org.uk>
To: linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC: akpm@linux-foundation.org, "Jiri Slaby" <jslaby@suse.cz>,
        "Takashi Iwai" <tiwai@suse.de>
Date: Wed, 27 Apr 2016 01:02:21 +0200
Message-ID: <lsq.1461711741.93779826@decadent.org.uk>
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Subject: [PATCH 3.16 164/217] ALSA: timer: Use mod_timer() for rearming
 the system timer
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3.16.35-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Takashi Iwai <tiwai@suse.de>

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.

Reported-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 sound/core/timer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -1053,8 +1053,8 @@ 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;
 }