sound: Implement asynchronous device detach

Hot-unplugging a sound device, such as a USB sound card, whilst being
consumed by an application, results in an infinite loop until either the
application closes the device's file descriptor, or the channel
automatically times out after hw.snd.timeout seconds. In the case of a
detach however, the timeout approach is still not ideal, since we want
all resources to be released immediatelly, without waiting for N seconds
until we can use the bus again.

The timeout mechanism works by calling chn_sleep() in chn_read() and
chn_write() (see pcm/channel.c) in order to send the thread to sleep,
using cv_timedwait_sig(). Since chn_sleep() sets the CHN_F_SLEEPING flag
while waiting for cv_timedwait_sig() to return, we can test this flag in
pcm_unregister() (called during detach) and wakeup the sleeping
thread(s) to immediately kill the channel(s) being consumed.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 months
PR:		194727, 278055, 202275, 220949, 272286
Reviewed by:	dev_submerge.ch, markj
Differential Revision:	https://reviews.freebsd.org/D43545

5 files changed, 15 insertions, 46 deletions

diff --git a/share/man/man4/snd_uaudio.4 b/share/man/man4/snd_uaudio.4
index b6a6c06a2312..6e2509b8f2ac 100644
--- a/share/man/man4/snd_uaudio.4
+++ b/share/man/man4/snd_uaudio.4
@@ -27,7 +27,7 @@
 .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 .\" POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd January 29, 2024
+.Dd March 26, 2024
 .Dt SND_UAUDIO 4
 .Os
 .Sh NAME
@@ -156,15 +156,6 @@ and modified for
 by
 .An Hiten Pandya Aq Mt hmp@FreeBSD.org .
 .Sh BUGS
-The PCM framework in
-.Fx
-only supports synchronous device detach.
-That means all mixer and DSP character devices belonging to a given
-USB audio device must be closed when receiving an error on a DSP read,
-a DSP write or a DSP IOCTL request.
-Else the USB audio driver will wait for this to happen, preventing
-enumeration of new devices on the parenting USB controller.
-.Pp
 Some USB audio devices might refuse to work properly unless the sample
 rate is configured the same for both recording and playback, even if
 only simplex is used.
diff --git a/sys/dev/sound/pcm/dsp.c b/sys/dev/sound/pcm/dsp.c
index b9693908da43..754f09c7199e 100644
--- a/sys/dev/sound/pcm/dsp.c
+++ b/sys/dev/sound/pcm/dsp.c
@@ -295,7 +295,7 @@ dsp_close(void *data)
 
 	d = priv->sc;
 	/* At this point pcm_unregister() will destroy all channels anyway. */
-	if (!PCM_REGISTERED(d))
+	if (PCM_DETACHING(d))
 		goto skip;
 
 	PCM_GIANT_ENTER(d);
diff --git a/sys/dev/sound/pcm/mixer.c b/sys/dev/sound/pcm/mixer.c
index ee1ed11a8ed0..cc8cf5b1ceea 100644
--- a/sys/dev/sound/pcm/mixer.c
+++ b/sys/dev/sound/pcm/mixer.c
@@ -817,17 +817,6 @@ mixer_uninit(device_t dev)
 	KASSERT(m->type == MIXER_TYPE_PRIMARY,
 	    ("%s(): illegal mixer type=%d", __func__, m->type));
 
-	snd_mtxlock(m->lock);
-
-	if (m->busy) {
-		snd_mtxunlock(m->lock);
-		return EBUSY;
-	}
-
-	/* destroy dev can sleep --hps */
-
-	snd_mtxunlock(m->lock);
-
 	pdev->si_drv1 = NULL;
 	destroy_dev(pdev);
 
diff --git a/sys/dev/sound/pcm/sound.c b/sys/dev/sound/pcm/sound.c
index 36a6f778e024..b45ca30dc984 100644
--- a/sys/dev/sound/pcm/sound.c
+++ b/sys/dev/sound/pcm/sound.c
@@ -1137,26 +1137,22 @@ pcm_unregister(device_t dev)
 
 	CHN_FOREACH(ch, d, channels.pcm) {
 		CHN_LOCK(ch);
-		if (ch->refcount > 0) {
-			device_printf(dev,
-			    "unregister: channel %s busy (pid %d)\n",
-			    ch->name, ch->pid);
-			CHN_UNLOCK(ch);
-			PCM_RELEASE_QUICK(d);
-			return (EBUSY);
+		if (ch->flags & CHN_F_SLEEPING) {
+			/*
+			 * We are detaching, so do not wait for the timeout in
+			 * chn_read()/chn_write(). Wake up the thread and kill
+			 * the channel immediately.
+			 */
+			CHN_BROADCAST(&ch->intr_cv);
+			ch->flags |= CHN_F_DEAD;
 		}
+		chn_abort(ch);
 		CHN_UNLOCK(ch);
 	}
 
 	dsp_destroy_dev(dev);
 
-	if (mixer_uninit(dev) == EBUSY) {
-		device_printf(dev, "unregister: mixer busy\n");
-		PCM_LOCK(d);
-		PCM_RELEASE(d);
-		PCM_UNLOCK(d);
-		return (EBUSY);
-	}
+	(void)mixer_uninit(dev);
 
 	/* remove /dev/sndstat entry first */
 	sndstat_unregister(dev);
diff --git a/sys/dev/sound/usb/uaudio.c b/sys/dev/sound/usb/uaudio.c
index 5d7396c527e0..2351c2522021 100644
--- a/sys/dev/sound/usb/uaudio.c
+++ b/sys/dev/sound/usb/uaudio.c
@@ -1255,20 +1255,13 @@ uaudio_detach_sub(device_t dev)
 	unsigned i = uaudio_get_child_index_by_dev(sc, dev);
 	int error = 0;
 
-repeat:
 	if (sc->sc_child[i].pcm_registered) {
 		error = pcm_unregister(dev);
-	} else {
-		if (sc->sc_child[i].mixer_init)
-			error = mixer_uninit(dev);
+	} else if (sc->sc_child[i].mixer_init) {
+		error = mixer_uninit(dev);
 	}
 
-	if (error) {
-		device_printf(dev, "Waiting for sound application to exit!\n");
-		usb_pause_mtx(NULL, 2 * hz);
-		goto repeat;		/* try again */
-	}
-	return (0);			/* success */
+	return (error);
 }
 
 static int