Merge branch '4383-limit-tree-pruning-overhead-9.18' into 'v9.18.22-release'

[9.18] Limit isc_task_send() overhead for tree pruning

See merge request isc-private/bind9!620

CHANGES

@@ -1,3 +1,6 @@
+6319.	[func]		Limit isc_task_send() overhead for RBTDB tree pruning.
+			[GL #4383]
+
 6317.	[security]	Restore DNS64 state when handling a serve-stale timeout.
 			(CVE-2023-5679) [GL #4334]
 

lib/dns/include/dns/rbt.h

@@ -124,6 +124,12 @@ struct dns_rbtnode {
 	 */
 	ISC_LINK(dns_rbtnode_t) deadlink;
 
+	/*%
+	 * This linked list is used to store nodes from which tree pruning can
+	 * be started.
+	 */
+	ISC_LINK(dns_rbtnode_t) prunelink;
+
 	/*@{*/
 	/*!
 	 * These values are used in the RBT DB implementation.  The appropriate

lib/dns/rbt.c

@@ -1576,6 +1576,7 @@ create_node(isc_mem_t *mctx, const dns_name_t *name, dns_rbtnode_t **nodep) {
 	HASHVAL(node) = 0;
 
 	ISC_LINK_INIT(node, deadlink);
+	ISC_LINK_INIT(node, prunelink);
 
 	LOCKNUM(node) = 0;
 	WILD(node) = 0;

lib/dns/rbtdb.c

@@ -494,6 +494,10 @@ struct dns_rbtdb {
 	 */
 	rbtnodelist_t *deadnodes;
 
+	/* List of nodes from which recursive tree pruning can be started from.
+	 * Locked by tree_lock. */
+	rbtnodelist_t prunenodes;
+
 	/*
 	 * Heaps.  These are used for TTL based expiry in a cache,
 	 * or for zone resigning in a zone DB.  hmctx is the memory
@@ -1003,6 +1007,7 @@ free_rbtdb(dns_rbtdb_t *rbtdb, bool log, isc_event_t *event) {
 	unsigned int i;
 	isc_result_t result;
 	char buf[DNS_NAME_FORMATSIZE];
+	dns_rbtnode_t *node = NULL;
 	dns_rbt_t **treep;
 	isc_time_t start;
 
@@ -1028,8 +1033,6 @@ free_rbtdb(dns_rbtdb_t *rbtdb, bool log, isc_event_t *event) {
 	 * the overhead of unlinking all nodes here should be negligible.
 	 */
 	for (i = 0; i < rbtdb->node_lock_count; i++) {
-		dns_rbtnode_t *node;
-
 		node = ISC_LIST_HEAD(rbtdb->deadnodes[i]);
 		while (node != NULL) {
 			ISC_LIST_UNLINK(rbtdb->deadnodes[i], node, deadlink);
@@ -1037,6 +1040,12 @@ free_rbtdb(dns_rbtdb_t *rbtdb, bool log, isc_event_t *event) {
 		}
 	}
 
+	node = ISC_LIST_HEAD(rbtdb->prunenodes);
+	while (node != NULL) {
+		ISC_LIST_UNLINK(rbtdb->prunenodes, node, prunelink);
+		node = ISC_LIST_HEAD(rbtdb->prunenodes);
+	}
+
 	if (event == NULL) {
 		rbtdb->quantum = (rbtdb->task != NULL) ? 100 : 0;
 	}
@@ -1842,19 +1851,32 @@ is_leaf(dns_rbtnode_t *node) {
 		node->left == NULL && node->right == NULL);
 }
 
+/*%
+ * The tree lock must be held when this function is called as it reads and
+ * updates rbtdb->prunenodes.
+ */
 static void
 send_to_prune_tree(dns_rbtdb_t *rbtdb, dns_rbtnode_t *node,
 		   isc_rwlocktype_t locktype) {
-	isc_event_t *ev;
-	dns_db_t *db;
+	bool pruning_queued = (ISC_LIST_HEAD(rbtdb->prunenodes) != NULL);
+
+	INSIST(locktype == isc_rwlocktype_write);
 
-	ev = isc_event_allocate(rbtdb->common.mctx, NULL, DNS_EVENT_RBTPRUNE,
-				prune_tree, node, sizeof(isc_event_t));
 	new_reference(rbtdb, node, locktype);
-	db = NULL;
-	attach((dns_db_t *)rbtdb, &db);
-	ev->ev_sender = db;
-	isc_task_send(rbtdb->task, &ev);
+	INSIST(!ISC_LINK_LINKED(node, prunelink));
+	ISC_LIST_APPEND(rbtdb->prunenodes, node, prunelink);
+
+	if (!pruning_queued) {
+		isc_event_t *ev = NULL;
+		dns_db_t *db = NULL;
+
+		attach((dns_db_t *)rbtdb, &db);
+
+		ev = isc_event_allocate(rbtdb->common.mctx, NULL,
+					DNS_EVENT_RBTPRUNE, prune_tree, db,
+					sizeof(isc_event_t));
+		isc_task_send(rbtdb->task, &ev);
+	}
 }
 
 /*%
@@ -2129,17 +2151,26 @@ restore_locks:
 }
 
 /*
- * Prune the tree by recursively cleaning-up single leaves.  In the worst
- * case, the number of iteration is the number of tree levels, which is at
- * most the maximum number of domain name labels, i.e, 127.  In practice, this
- * should be much smaller (only a few times), and even the worst case would be
- * acceptable for a single event.
+ * Prune the tree by recursively cleaning up single leaves.  Go through all
+ * nodes stored in the rbtdb->prunenodes list; for each of them, in the worst
+ * case, it will be necessary to traverse a number of tree levels equal to the
+ * maximum legal number of domain name labels (127); in practice, the number of
+ * tree levels to traverse will virtually always be much smaller (a few levels
+ * at most).  While holding the tree lock throughout this entire operation is
+ * less than ideal, so is splitting the latter up by queueing a separate
+ * prune_tree() run for each node to start pruning from (as queueing requires
+ * allocating memory and can therefore potentially be exploited to exhaust
+ * available memory).  Also note that actually freeing up the memory used by
+ * RBTDB nodes (which is what this function does) is essential to keeping cache
+ * memory use in check, so since the tree lock needs to be acquired anyway,
+ * freeing as many nodes as possible before the tree lock gets released is
+ * prudent.
  */
 static void
 prune_tree(isc_task_t *task, isc_event_t *event) {
-	dns_rbtdb_t *rbtdb = event->ev_sender;
-	dns_rbtnode_t *node = event->ev_arg;
-	dns_rbtnode_t *parent;
+	dns_rbtdb_t *rbtdb = (dns_rbtdb_t *)event->ev_arg;
+	dns_rbtnode_t *node = NULL;
+	dns_rbtnode_t *parent = NULL;
 	unsigned int locknum;
 
 	UNUSED(task);
@@ -2147,44 +2178,60 @@ prune_tree(isc_task_t *task, isc_event_t *event) {
 	isc_event_free(&event);
 
 	RWLOCK(&rbtdb->tree_lock, isc_rwlocktype_write);
-	locknum = node->locknum;
-	NODE_LOCK(&rbtdb->node_locks[locknum].lock, isc_rwlocktype_write);
-	do {
-		parent = node->parent;
-		decrement_reference(rbtdb, node, 0, isc_rwlocktype_write,
-				    isc_rwlocktype_write, true);
 
-		if (parent != NULL && parent->down == NULL) {
-			/*
-			 * node was the only down child of the parent and has
-			 * just been removed.  We'll then need to examine the
-			 * parent.  Keep the lock if possible; otherwise,
-			 * release the old lock and acquire one for the parent.
-			 */
-			if (parent->locknum != locknum) {
-				NODE_UNLOCK(&rbtdb->node_locks[locknum].lock,
-					    isc_rwlocktype_write);
-				locknum = parent->locknum;
-				NODE_LOCK(&rbtdb->node_locks[locknum].lock,
-					  isc_rwlocktype_write);
+	while ((node = ISC_LIST_HEAD(rbtdb->prunenodes)) != NULL) {
+		locknum = node->locknum;
+		NODE_LOCK(&rbtdb->node_locks[locknum].lock,
+			  isc_rwlocktype_write);
+		do {
+			if (ISC_LINK_LINKED(node, prunelink)) {
+				ISC_LIST_UNLINK(rbtdb->prunenodes, node,
+						prunelink);
 			}
 
-			/*
-			 * We need to gain a reference to the node before
-			 * decrementing it in the next iteration.
-			 */
-			if (ISC_LINK_LINKED(parent, deadlink)) {
-				ISC_LIST_UNLINK(rbtdb->deadnodes[locknum],
+			parent = node->parent;
+			decrement_reference(rbtdb, node, 0,
+					    isc_rwlocktype_write,
+					    isc_rwlocktype_write, true);
+
+			if (parent != NULL && parent->down == NULL) {
+				/*
+				 * node was the only down child of the parent
+				 * and has just been removed.  We'll then need
+				 * to examine the parent.  Keep the lock if
+				 * possible; otherwise, release the old lock and
+				 * acquire one for the parent.
+				 */
+				if (parent->locknum != locknum) {
+					NODE_UNLOCK(
+						&rbtdb->node_locks[locknum].lock,
+						isc_rwlocktype_write);
+					locknum = parent->locknum;
+					NODE_LOCK(
+						&rbtdb->node_locks[locknum].lock,
+						isc_rwlocktype_write);
+				}
+
+				/*
+				 * We need to gain a reference to the node
+				 * before decrementing it in the next iteration.
+				 */
+				if (ISC_LINK_LINKED(parent, deadlink)) {
+					ISC_LIST_UNLINK(
+						rbtdb->deadnodes[locknum],
 						parent, deadlink);
+				}
+				new_reference(rbtdb, parent,
+					      isc_rwlocktype_write);
+			} else {
+				parent = NULL;
 			}
-			new_reference(rbtdb, parent, isc_rwlocktype_write);
-		} else {
-			parent = NULL;
-		}
 
-		node = parent;
-	} while (node != NULL);
-	NODE_UNLOCK(&rbtdb->node_locks[locknum].lock, isc_rwlocktype_write);
+			node = parent;
+		} while (node != NULL);
+		NODE_UNLOCK(&rbtdb->node_locks[locknum].lock,
+			    isc_rwlocktype_write);
+	}
 	RWUNLOCK(&rbtdb->tree_lock, isc_rwlocktype_write);
 
 	detach((dns_db_t **)&rbtdb);
@@ -8326,6 +8373,8 @@ dns_rbtdb_create(isc_mem_t *mctx, const dns_name_t *origin, dns_dbtype_t type,
 		ISC_LIST_INIT(rbtdb->deadnodes[i]);
 	}
 
+	ISC_LIST_INIT(rbtdb->prunenodes);
+
 	rbtdb->active = rbtdb->node_lock_count;
 
 	for (i = 0; i < (int)(rbtdb->node_lock_count); i++) {