ALSA: timer: Handle disconnection more safely

Currently ALSA timer device doesn't take the disconnection into
account very well; it merely unlinks the timer device at disconnection
callback but does nothing else.  Because of this, when an application
accessing the timer device is disconnected, it may release the
resource before actually closed.  In most cases, it results in a
warning message indicating a leftover timer instance like:
   ALSA: timer xxxx is busy?
But basically this is an open race.

This patch tries to address it.  The strategy is like other ALSA
devices: namely,
- Manage card's refcount at each open/close
- Wake up the pending tasks at disconnection
- Check the shutdown flag appropriately at each possible call

Note that this patch has one ugly hack to handle the wakeup of pending
tasks.  It'd be cleaner to introduce a new disconnect op to
snd_timer_instance ops.  But since it would lead to internal ABI
breakage and it eventually increase my own work when backporting to
stable kernels, I took a different path to implement locally in
timer.c.  A cleanup patch will follow at next for 4.5 kernel.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=109431
Cc: <stable@vger.kernel.org> # v3.15+
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/timer.c

@@ -65,6 +65,7 @@ struct snd_timer_user {
 	int qtail;
 	int qused;
 	int queue_size;
+	bool disconnected;
 	struct snd_timer_read *queue;
 	struct snd_timer_tread *tqueue;
 	spinlock_t qlock;
@@ -290,6 +291,9 @@ int snd_timer_open(struct snd_timer_instance **ti,
 		mutex_unlock(&register_mutex);
 		return -ENOMEM;
 	}
+	/* take a card refcount for safe disconnection */
+	if (timer->card)
+		get_device(&timer->card->card_dev);
 	timeri->slave_class = tid->dev_sclass;
 	timeri->slave_id = slave_id;
 	if (list_empty(&timer->open_list_head) && timer->hw.open)
@@ -359,6 +363,9 @@ int snd_timer_close(struct snd_timer_instance *timeri)
 		}
 		spin_unlock(&timer->lock);
 		spin_unlock_irq(&slave_active_lock);
+		/* release a card refcount for safe disconnection */
+		if (timer->card)
+			put_device(&timer->card->card_dev);
 		mutex_unlock(&register_mutex);
 	}
  out:
@@ -474,6 +481,8 @@ int snd_timer_start(struct snd_timer_instance *timeri, unsigned int ticks)
 	timer = timeri->timer;
 	if (timer == NULL)
 		return -EINVAL;
+	if (timer->card && timer->card->shutdown)
+		return -ENODEV;
 	spin_lock_irqsave(&timer->lock, flags);
 	timeri->ticks = timeri->cticks = ticks;
 	timeri->pticks = 0;
@@ -505,6 +514,10 @@ static int _snd_timer_stop(struct snd_timer_instance *timeri, int event)
 	spin_lock_irqsave(&timer->lock, flags);
 	list_del_init(&timeri->ack_list);
 	list_del_init(&timeri->active_list);
+	if (timer->card && timer->card->shutdown) {
+		spin_unlock_irqrestore(&timer->lock, flags);
+		return 0;
+	}
 	if ((timeri->flags & SNDRV_TIMER_IFLG_RUNNING) &&
 	    !(--timer->running)) {
 		timer->hw.stop(timer);
@@ -565,6 +578,8 @@ int snd_timer_continue(struct snd_timer_instance *timeri)
 	timer = timeri->timer;
 	if (! timer)
 		return -EINVAL;
+	if (timer->card && timer->card->shutdown)
+		return -ENODEV;
 	spin_lock_irqsave(&timer->lock, flags);
 	if (!timeri->cticks)
 		timeri->cticks = 1;
@@ -628,6 +643,9 @@ static void snd_timer_tasklet(unsigned long arg)
 	unsigned long resolution, ticks;
 	unsigned long flags;
 
+	if (timer->card && timer->card->shutdown)
+		return;
+
 	spin_lock_irqsave(&timer->lock, flags);
 	/* now process all callbacks */
 	while (!list_empty(&timer->sack_list_head)) {
@@ -668,6 +686,9 @@ void snd_timer_interrupt(struct snd_timer * timer, unsigned long ticks_left)
 	if (timer == NULL)
 		return;
 
+	if (timer->card && timer->card->shutdown)
+		return;
+
 	spin_lock_irqsave(&timer->lock, flags);
 
 	/* remember the current resolution */
@@ -878,11 +899,28 @@ static int snd_timer_dev_register(struct snd_device *dev)
 	return 0;
 }
 
+/* just for reference in snd_timer_dev_disconnect() below */
+static void snd_timer_user_ccallback(struct snd_timer_instance *timeri,
+				     int event, struct timespec *tstamp,
+				     unsigned long resolution);
+
 static int snd_timer_dev_disconnect(struct snd_device *device)
 {
 	struct snd_timer *timer = device->device_data;
+	struct snd_timer_instance *ti;
+
 	mutex_lock(&register_mutex);
 	list_del_init(&timer->device_list);
+	/* wake up pending sleepers */
+	list_for_each_entry(ti, &timer->open_list_head, open_list) {
+		/* FIXME: better to have a ti.disconnect() op */
+		if (ti->ccallback == snd_timer_user_ccallback) {
+			struct snd_timer_user *tu = ti->callback_data;
+
+			tu->disconnected = true;
+			wake_up(&tu->qchange_sleep);
+		}
+	}
 	mutex_unlock(&register_mutex);
 	return 0;
 }
@@ -893,6 +931,8 @@ void snd_timer_notify(struct snd_timer *timer, int event, struct timespec *tstam
 	unsigned long resolution = 0;
 	struct snd_timer_instance *ti, *ts;
 
+	if (timer->card && timer->card->shutdown)
+		return;
 	if (! (timer->hw.flags & SNDRV_TIMER_HW_SLAVE))
 		return;
 	if (snd_BUG_ON(event < SNDRV_TIMER_EVENT_MSTART ||
@@ -1051,6 +1091,8 @@ static void snd_timer_proc_read(struct snd_info_entry *entry,
 
 	mutex_lock(&register_mutex);
 	list_for_each_entry(timer, &snd_timer_list, device_list) {
+		if (timer->card && timer->card->shutdown)
+			continue;
 		switch (timer->tmr_class) {
 		case SNDRV_TIMER_CLASS_GLOBAL:
 			snd_iprintf(buffer, "G%i: ", timer->tmr_device);
@@ -1876,6 +1918,10 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
 
 			remove_wait_queue(&tu->qchange_sleep, &wait);
 
+			if (tu->disconnected) {
+				err = -ENODEV;
+				break;
+			}
 			if (signal_pending(current)) {
 				err = -ERESTARTSYS;
 				break;
@@ -1925,6 +1971,8 @@ static unsigned int snd_timer_user_poll(struct file *file, poll_table * wait)
 	mask = 0;
 	if (tu->qused)
 		mask |= POLLIN | POLLRDNORM;
+	if (tu->disconnected)
+		mask |= POLLERR;
 
 	return mask;
 }