diff --git a/lib/isc/rwlock.c b/lib/isc/rwlock.c
index 3b3b342627..5b3ce61f60 100644
--- a/lib/isc/rwlock.c
+++ b/lib/isc/rwlock.c
@@ -1,4 +1,6 @@
 
+#include <stdio.h>
+
 #include <isc/assertions.h>
 #include <isc/unexpect.h>
 #include <isc/boolean.h>
@@ -15,21 +17,23 @@
 #define WAIT(cvp, lp) \
 	INSIST(isc_condition_wait((cvp), (lp)) == ISC_R_SUCCESS);
 
-#include <stdio.h>
+#define RWLOCK_MAGIC			0x52574C6BU	/* RWLk. */
+#define VALID_RWLOCK(rwl)		((rwl) != NULL && \
+					 (rwl)->magic == RWLOCK_MAGIC)
 
+#ifdef ISC_RWLOCK_TRACE
 static void
 print_lock(char *operation, isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	printf("%s(%s):  ", operation,
 	       (type == isc_rwlocktype_read ? "read" : "write"));
-	printf("%s, %u active",
+	printf("%s, %u active, %u granted",
 	       (rwl->type == isc_rwlocktype_read ? "reading" : "writing"),
-	       rwl->active);
-	if (rwl->type == isc_rwlocktype_read)
-		printf(", %u granted", rwl->granted);
+	       rwl->active, rwl->granted);
 	printf(", %u rwaiting, %u wwaiting\n",
 	       rwl->readers_waiting,
 	       rwl->writers_waiting);
 }
+#endif
 
 isc_result_t
 isc_rwlock_init(isc_rwlock_t *rwl,
@@ -40,6 +44,12 @@ isc_rwlock_init(isc_rwlock_t *rwl,
 
 	REQUIRE(rwl != NULL);
 
+	/*
+	 * In case there's trouble initializing, we zero magic now.  If all
+	 * goes well, we'll set it to RWLOCK_MAGIC.
+	 */
+	rwl->magic = 0;
+
 	rwl->type = isc_rwlocktype_read;
 	rwl->active = 0;
 	rwl->granted = 0;
@@ -73,6 +83,8 @@ isc_rwlock_init(isc_rwlock_t *rwl,
 		return (ISC_R_UNEXPECTED);
 	}
 
+	rwl->magic = RWLOCK_MAGIC;
+
 	return (ISC_R_SUCCESS);
 }
 
@@ -81,9 +93,11 @@ isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	isc_boolean_t skip = ISC_FALSE;
 	isc_boolean_t done = ISC_FALSE;
 
+	REQUIRE(VALID_RWLOCK(rwl));
+
 	LOCK(&rwl->lock);
 
-#ifdef DEBUG
+#ifdef ISC_RWLOCK_TRACE
 	print_lock("prelock", rwl, type);
 #endif
 
@@ -124,7 +138,7 @@ isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 		}
 	}
 
-#ifdef DEBUG
+#ifdef ISC_RWLOCK_TRACE
 	print_lock("postlock", rwl, type);
 #endif
 
@@ -135,10 +149,12 @@ isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 
 isc_result_t
 isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+
+	REQUIRE(VALID_RWLOCK(rwl));
 	LOCK(&rwl->lock);
 	REQUIRE(rwl->type == type);
 
-#ifdef DEBUG
+#ifdef ISC_RWLOCK_TRACE
 	print_lock("preunlock", rwl, type);
 #endif
 
@@ -166,6 +182,8 @@ isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 			} else if (rwl->writers_waiting > 0) {
 				rwl->granted = 0;
 				SIGNAL(&rwl->writeable);
+			} else {
+				rwl->granted = 0;
 			}
 		}
 	} else {
@@ -178,7 +196,7 @@ isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 		}
 	}
 
-#ifdef DEBUG
+#ifdef ISC_RWLOCK_TRACE
 	print_lock("postunlock", rwl, type);
 #endif
 
@@ -189,11 +207,15 @@ isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 
 void
 isc_rwlock_destroy(isc_rwlock_t *rwl) {
+
+	REQUIRE(VALID_RWLOCK(rwl));
 	LOCK(&rwl->lock);
 	REQUIRE(rwl->active == 0 &&
 		rwl->readers_waiting == 0 &&
 		rwl->writers_waiting == 0);
 	UNLOCK(&rwl->lock);
+
+	rwl->magic = 0;
 	(void)isc_condition_destroy(&rwl->readable);
 	(void)isc_condition_destroy(&rwl->writeable);
 	(void)isc_mutex_destroy(&rwl->lock);