sound: Fix hot-unload panics

This patch fixes multiple different panic scenarios occuring during
hot-unload:

1. The channel is unlocked in chn_read()/chn_write() for uiomove(9) and
   in the meantime we enter pcm_killchans() and free it. By the time we
   have returned from userland and try to lock it back, the channel will
   have been freed.
2. The parent channel has been freed in pcm_killchans(), but at the same
   time, some yet-unstopped vchan's chn_read()/chn_write() calls
   chn_start(), which eventually calls vchan_trigger(), which references
   the freed parent.
3. PCM_WAIT() panics because it references a freed PCM lock.

For scenarios 1 and 2, refactor pcm_killchans() to first make sure all
channels have been stopped, and then proceed to free them one by one, as
opposed to freeing the first free channel until all channels have been
freed. This change makes the code more robust, but might introduce some
performance overhead when many channels are allocated, since we
continuously loop through the channel list until all of them are
stopped, and then we loop one last time to free them.

For scenario 3, restructure the code so that we can use destroy_dev(9)
instead of destroy_dev_sched(9) in dsp_destroy_dev(). Because
destroy_dev(9) blocks until all references to the device have went away,
we ensure that the PCM cv and lock will be freed safely.

While here, move the delete_unrhdr(9) calls to pcm_killchans() and
re-order some lines.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 days
Reviewed by:	dev_submerge.ch
Differential Revision:	https://reviews.freebsd.org/D47462

sys/dev/sound/pcm/dsp.c

@@ -137,7 +137,7 @@ dsp_destroy_dev(device_t dev)
 	struct snddev_info *d;
 
 	d = device_get_softc(dev);
-	destroy_dev_sched(d->dsp_dev);
+	destroy_dev(d->dsp_dev);
 }
 
 static void

sys/dev/sound/pcm/sound.c

@@ -211,40 +211,53 @@ static void
 pcm_killchans(struct snddev_info *d)
 {
 	struct pcm_channel *ch;
-	bool found;
+	bool again;
 
 	PCM_BUSYASSERT(d);
-	do {
-		found = false;
+	KASSERT(!PCM_REGISTERED(d), ("%s(): still registered\n", __func__));
+
+	for (;;) {
+		again = false;
+		/* Make sure all channels are stopped. */
 		CHN_FOREACH(ch, d, channels.pcm) {
 			CHN_LOCK(ch);
-			/*
-			 * Make sure no channel has went to sleep in the
-			 * meantime.
-			 */
-			chn_shutdown(ch);
-			/*
-			 * We have to give a thread sleeping in chn_sleep() a
-			 * chance to observe that the channel is dead.
-			 */
-			if ((ch->flags & CHN_F_SLEEPING) == 0) {
-				found = true;
+			if ((ch->flags & CHN_F_SLEEPING) == 0 &&
+			    CHN_STOPPED(ch) && ch->inprog == 0) {
 				CHN_UNLOCK(ch);
-				break;
+				continue;
 			}
+			chn_shutdown(ch);
+			if (ch->direction == PCMDIR_PLAY)
+				chn_flush(ch);
+			else
+				chn_abort(ch);
 			CHN_UNLOCK(ch);
+			again = true;
 		}
-
 		/*
-		 * All channels are still sleeping. Sleep for a bit and try
-		 * again to see if any of them is awake now.
+		 * Some channels are still active. Sleep for a bit and try
+		 * again.
 		 */
-		if (!found) {
-			pause_sbt("pcmkillchans", SBT_1MS * 5, 0, 0);
-			continue;
-		}
+		if (again)
+			pause_sbt("pcmkillchans", mstosbt(5), 0, 0);
+		else
+			break;
+	}
+
+	/* All channels are finally dead. */
+	while (!CHN_EMPTY(d, channels.pcm)) {
+		ch = CHN_FIRST(d, channels.pcm);
 		chn_kill(ch);
-	} while (!CHN_EMPTY(d, channels.pcm));
+	}
+
+	if (d->p_unr != NULL)
+		delete_unrhdr(d->p_unr);
+	if (d->vp_unr != NULL)
+		delete_unrhdr(d->vp_unr);
+	if (d->r_unr != NULL)
+		delete_unrhdr(d->r_unr);
+	if (d->vr_unr != NULL)
+		delete_unrhdr(d->vr_unr);
 }
 
 static int
@@ -512,7 +525,6 @@ int
 pcm_unregister(device_t dev)
 {
 	struct snddev_info *d;
-	struct pcm_channel *ch;
 
 	d = device_get_softc(dev);
 
@@ -525,28 +537,15 @@ pcm_unregister(device_t dev)
 	PCM_WAIT(d);
 
 	d->flags |= SD_F_DETACHING;
+	d->flags |= SD_F_DYING;
+	d->flags &= ~SD_F_REGISTERED;
 
 	PCM_ACQUIRE(d);
 	PCM_UNLOCK(d);
 
-	CHN_FOREACH(ch, d, channels.pcm) {
-		CHN_LOCK(ch);
-		/*
-		 * Do not wait for the timeout in chn_read()/chn_write(). Wake
-		 * up the sleeping thread and kill the channel.
-		 */
-		chn_shutdown(ch);
-		chn_abort(ch);
-		CHN_UNLOCK(ch);
-	}
+	pcm_killchans(d);
 
-	/* remove /dev/sndstat entry first */
-	sndstat_unregister(dev);
-
-	PCM_LOCK(d);
-	d->flags |= SD_F_DYING;
-	d->flags &= ~SD_F_REGISTERED;
-	PCM_UNLOCK(d);
+	PCM_RELEASE_QUICK(d);
 
 	if (d->play_sysctl_tree != NULL) {
 		sysctl_ctx_free(&d->play_sysctl_ctx);
@@ -557,24 +556,12 @@ pcm_unregister(device_t dev)
 		d->rec_sysctl_tree = NULL;
 	}
 
+	sndstat_unregister(dev);
+	mixer_uninit(dev);
 	dsp_destroy_dev(dev);
-	(void)mixer_uninit(dev);
 
-	pcm_killchans(d);
-
-	PCM_LOCK(d);
-	PCM_RELEASE(d);
 	cv_destroy(&d->cv);
-	PCM_UNLOCK(d);
 	snd_mtxfree(d->lock);
-	if (d->p_unr != NULL)
-		delete_unrhdr(d->p_unr);
-	if (d->vp_unr != NULL)
-		delete_unrhdr(d->vp_unr);
-	if (d->r_unr != NULL)
-		delete_unrhdr(d->r_unr);
-	if (d->vr_unr != NULL)
-		delete_unrhdr(d->vr_unr);
 
 	if (snd_unit == device_get_unit(dev)) {
 		snd_unit = pcm_best_unit(-1);