Index: zaptel.c
===================================================================
--- zaptel.c	(revision 1253)
+++ zaptel.c	(working copy)
@@ -1091,8 +1091,14 @@
 	/* Assumes channel is already locked */
 	if ((zone >= ZT_TONE_ZONE_MAX) || (zone < -1))
 		return -EINVAL;
-	
-	read_lock(&zone_lock);
+
+	/* Since this routine is called both from IRQ as well as from userspace,
+	 * it is possible that we could be called during an IRQ while userspace
+	 * has locked this.  However unlikely, this could possibly cause a
+	 * deadlock. */
+	if (! read_trylock(&zone_lock))
+		return -EWOULDBLOCK;
+
 	if (zone == -1) {
 		zone = default_zone;
 	}
@@ -1103,7 +1109,7 @@
 	} else {
 		res = -ENODATA;
 	}
-	
+
 	read_unlock(&zone_lock);
 	return res;
 }