linux-kernel - [ 111/147] ALSA: aloop - add locking to timer access

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Message-Id: <20121014143548.761520731@decadent.org.uk>
Date:	Sun, 14 Oct 2012 15:37:24 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Omair Mohammed Abdullah <omair.m.abdullah@...ux.intel.com>,
	Vinod Koul <vinod.koul@...ux.intel.com>,
	Takashi Iwai <tiwai@...e.de>
Subject: [ 111/147] ALSA: aloop - add locking to timer access

3.2-stable review patch.  If anyone has any objections, please let me know.

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

From: Omair Mohammed Abdullah <omair.m.abdullah@...ux.intel.com>

commit d4f1e48bd11e3df6a26811f7a1f06c4225d92f7d upstream.

When the loopback timer handler is running, calling del_timer() (for STOP
trigger) will not wait for the handler to complete before deactivating the
timer. The timer gets rescheduled in the handler as usual. Then a subsequent
START trigger will try to start the timer using add_timer() with a timer pending
leading to a kernel panic.

Serialize the calls to add_timer() and del_timer() using a spin lock to avoid
this.

Signed-off-by: Omair Mohammed Abdullah <omair.m.abdullah@...ux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@...ux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 sound/drivers/aloop.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/sound/drivers/aloop.c
+++ b/sound/drivers/aloop.c
@@ -119,6 +119,7 @@ struct loopback_pcm {
 	unsigned int period_size_frac;
 	unsigned long last_jiffies;
 	struct timer_list timer;
+	spinlock_t timer_lock;
 };
 
 static struct platform_device *devices[SNDRV_CARDS];
@@ -169,6 +170,7 @@ static void loopback_timer_start(struct
 	unsigned long tick;
 	unsigned int rate_shift = get_rate_shift(dpcm);
 
+	spin_lock(&dpcm->timer_lock);
 	if (rate_shift != dpcm->pcm_rate_shift) {
 		dpcm->pcm_rate_shift = rate_shift;
 		dpcm->period_size_frac = frac_pos(dpcm, dpcm->pcm_period_size);
@@ -181,12 +183,15 @@ static void loopback_timer_start(struct
 	tick = (tick + dpcm->pcm_bps - 1) / dpcm->pcm_bps;
 	dpcm->timer.expires = jiffies + tick;
 	add_timer(&dpcm->timer);
+	spin_unlock(&dpcm->timer_lock);
 }
 
 static inline void loopback_timer_stop(struct loopback_pcm *dpcm)
 {
+	spin_lock(&dpcm->timer_lock);
 	del_timer(&dpcm->timer);
 	dpcm->timer.expires = 0;
+	spin_unlock(&dpcm->timer_lock);
 }
 
 #define CABLE_VALID_PLAYBACK	(1 << SNDRV_PCM_STREAM_PLAYBACK)
@@ -659,6 +664,7 @@ static int loopback_open(struct snd_pcm_
 	dpcm->substream = substream;
 	setup_timer(&dpcm->timer, loopback_timer_function,
 		    (unsigned long)dpcm);
+	spin_lock_init(&dpcm->timer_lock);
 
 	cable = loopback->cables[substream->number][dev];
 	if (!cable) {


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