diff --git a/CHANGES b/CHANGES
index 8b72400507..6b95740f91 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,5 @@
+1461.	[bug]		Remove deadlock from rbtdb code. [RT #5999]
+
 1460.	[bug]		inet_pton() failed to reject certian malformed
 			IPv6 literals.
 
diff --git a/lib/dns/rbtdb.c b/lib/dns/rbtdb.c
index 4ec6a9e185..267f5cc2bc 100644
--- a/lib/dns/rbtdb.c
+++ b/lib/dns/rbtdb.c
@@ -15,7 +15,7 @@
  * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: rbtdb.c,v 1.186 2003/02/27 00:19:03 marka Exp $ */
+/* $Id: rbtdb.c,v 1.187 2003/04/17 01:56:34 marka Exp $ */
 
 /*
  * Principal Author: Bob Halley
@@ -815,10 +815,10 @@ no_references(dns_rbtdb_t *rbtdb, dns_rbtnode_t *node,
 		 * we only do a trylock.
 		 */
 		if (lock == isc_rwlocktype_read)
-			RWUNLOCK(&rbtdb->tree_lock, isc_rwlocktype_read);
-
-		result = isc_rwlock_trylock(&rbtdb->tree_lock,
-					    isc_rwlocktype_write);
+			result = isc_rwlock_tryupgrade(&rbtdb->tree_lock);
+		else
+			result = isc_rwlock_trylock(&rbtdb->tree_lock,
+						    isc_rwlocktype_write);
 		RUNTIME_CHECK(result == ISC_R_SUCCESS ||
 			      result == ISC_R_LOCKBUSY);
  
@@ -849,12 +849,13 @@ no_references(dns_rbtdb_t *rbtdb, dns_rbtnode_t *node,
 	/*
 	 * Relock a read lock, or unlock the write lock if no lock was held.
 	 */
-	if (lock != isc_rwlocktype_write)
+	if (lock == isc_rwlocktype_none)
 		if (write_locked)
 			RWUNLOCK(&rbtdb->tree_lock, isc_rwlocktype_write);
 
 	if (lock == isc_rwlocktype_read)
-		RWLOCK(&rbtdb->tree_lock, isc_rwlocktype_read);
+		if (write_locked)
+			isc_rwlock_downgrade(&rbtdb->tree_lock);
 }
 
 static inline void
diff --git a/lib/isc/include/isc/rwlock.h b/lib/isc/include/isc/rwlock.h
index 0313c050c2..d84d4db210 100644
--- a/lib/isc/include/isc/rwlock.h
+++ b/lib/isc/include/isc/rwlock.h
@@ -15,7 +15,7 @@
  * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: rwlock.h,v 1.18 2001/03/08 00:55:15 tale Exp $ */
+/* $Id: rwlock.h,v 1.19 2003/04/17 01:56:35 marka Exp $ */
 
 #ifndef ISC_RWLOCK_H
 #define ISC_RWLOCK_H 1
@@ -80,6 +80,12 @@ isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type);
 isc_result_t
 isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type);
 
+isc_result_t
+isc_rwlock_tryupgrade(isc_rwlock_t *rwl);
+
+void
+isc_rwlock_downgrade(isc_rwlock_t *rwl);
+
 void
 isc_rwlock_destroy(isc_rwlock_t *rwl);
 
diff --git a/lib/isc/rwlock.c b/lib/isc/rwlock.c
index 6ddcbdba6b..25172dcdd3 100644
--- a/lib/isc/rwlock.c
+++ b/lib/isc/rwlock.c
@@ -15,7 +15,7 @@
  * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-/* $Id: rwlock.c,v 1.33 2001/04/17 14:36:45 tale Exp $ */
+/* $Id: rwlock.c,v 1.34 2003/04/17 01:56:34 marka Exp $ */
 
 #include <config.h>
 
@@ -205,6 +205,44 @@ isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	return (doit(rwl, type, ISC_TRUE));
 }
 
+isc_result_t
+isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
+	isc_result_t result = ISC_R_SUCCESS;
+
+	REQUIRE(VALID_RWLOCK(rwl));
+	LOCK(&rwl->lock);
+	REQUIRE(rwl->type == isc_rwlocktype_read);
+	REQUIRE(rwl->active != 0);
+
+	/* If we are the only reader then succeed. */
+	if (rwl->active == 1)
+		rwl->type = isc_rwlocktype_write;
+	else
+		result = ISC_R_LOCKBUSY;
+
+	UNLOCK(&rwl->lock);
+	return (result);
+}
+
+void
+isc_rwlock_downgrade(isc_rwlock_t *rwl) {
+
+	REQUIRE(VALID_RWLOCK(rwl));
+	LOCK(&rwl->lock);
+	REQUIRE(rwl->type == isc_rwlocktype_write);
+	REQUIRE(rwl->active == 1);
+
+	rwl->type = isc_rwlocktype_read;
+	/*
+	 * Wake up waiting readers if there are no waiting writers.
+	 */
+	if ((rwl->writers_waiting == 0 || rwl->granted < rwl->read_quota) &&
+	    rwl->readers_waiting > 0)
+		BROADCAST(&rwl->readable);
+
+	UNLOCK(&rwl->lock);
+}
+
 isc_result_t
 isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 
@@ -318,6 +356,32 @@ isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	return (isc_rwlock_lock(rwl, type));
 }
 
+isc_result_t
+isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
+	isc_result_t result = ISC_R_SUCCESS;
+
+	REQUIRE(VALID_RWLOCK(rwl));
+	REQUIRE(rwl->type == isc_rwlocktype_read);
+	REQUIRE(rwl->active != 0);
+	
+	/* If we are the only reader then succeed. */
+	if (rwl->active == 1)
+		rwl->type = isc_rwlocktype_write;
+	else
+		result = ISC_R_LOCKBUSY;
+	return (result);
+}
+
+void
+isc_rwlock_downgrade(isc_rwlock_t *rwl) {
+
+	REQUIRE(VALID_RWLOCK(rwl));
+	REQUIRE(rwl->type == isc_rwlocktype_write);
+	REQUIRE(rwl->active == 1);
+
+	rwl->type = isc_rwlocktype_read;
+}
+
 isc_result_t
 isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	REQUIRE(VALID_RWLOCK(rwl));