sound: Fix lock order reversal in dsp_poll()

chn_poll() may hold both rdch and wrch channel locks while calling
chn_trigger(rdch).  chn_trigger() switches the lock order from
"channel -> dsp dev" to "dsp dev -> channel" by temporarily dropping
the channel lock before acquiring the dsp lock.

However, only rdch was unlocked during the transition while wrch
remained locked. Since wrch is also a channel lock and witness had
already established the lock order requirement:

    dsp dev -> channel

witness reports a lock order reversal when pcm_lock() is acquired while
wrch is still held.

Avoid holding rdch and wrch simultaneously during chn_trigger()
lock-order switching by only keeping the channel locks when needed.

The issue can be reliably reproduced by starting pipewire,
pipewire-pulse, and pavucontrol.

Reviewed by:    christos
MFC after:      2 weeks
Sponsored by:   The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57009

sys/dev/sound/pcm/dsp.c

@@ -1877,24 +1877,25 @@ dsp_poll(struct cdev *i_dev, int events, struct thread *td)
 
 	ret = 0;
 
-	dsp_lock_chans(priv, FREAD | FWRITE);
 	wrch = priv->wrch;
 	rdch = priv->rdch;
 
 	if (wrch != NULL && !(wrch->flags & CHN_F_DEAD)) {
+		CHN_LOCK(wrch);
 		e = (events & (POLLOUT | POLLWRNORM));
 		if (e)
 			ret |= chn_poll(wrch, e, td);
+		CHN_UNLOCK(wrch);
 	}
 
 	if (rdch != NULL && !(rdch->flags & CHN_F_DEAD)) {
+		CHN_LOCK(rdch);
 		e = (events & (POLLIN | POLLRDNORM));
 		if (e)
 			ret |= chn_poll(rdch, e, td);
+		CHN_UNLOCK(rdch);
 	}
 
-	dsp_unlock_chans(priv, FREAD | FWRITE);
-
 	PCM_GIANT_LEAVE(d);
 
 	return (ret);