Drop locks in btrfs_search_slot when reading a tree block.

One lock per btree block can make for significant congestion if everyone
has to wait for IO at the high levels of the btree.  This drops
locks held by a path when doing reads during a tree search.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.c

@@ -63,7 +63,6 @@ void btrfs_free_path(struct btrfs_path *p)
 void btrfs_release_path(struct btrfs_root *root, struct btrfs_path *p)
 {
 	int i;
-	int skip = p->skip_locking;
 	int keep = p->keep_locks;
 
 	for (i = 0; i < BTRFS_MAX_LEVEL; i++) {
@@ -76,7 +75,6 @@ void btrfs_release_path(struct btrfs_root *root, struct btrfs_path *p)
 		free_extent_buffer(p->nodes[i]);
 	}
 	memset(p, 0, sizeof(*p));
-	p->skip_locking = skip;
 	p->keep_locks = keep;
 }
 
@@ -1137,7 +1135,6 @@ static void reada_for_search(struct btrfs_root *root, struct btrfs_path *path,
 		return;
 
 	node = path->nodes[level];
-	WARN_ON(!path->skip_locking && !btrfs_tree_locked(node));
 
 	search = btrfs_node_blockptr(node, slot);
 	blocksize = btrfs_level_size(root, level - 1);
@@ -1192,6 +1189,7 @@ static void unlock_up(struct btrfs_path *path, int level, int lowest_unlock)
 {
 	int i;
 	int skip_level = level;
+	int no_skips = 0;
 	struct extent_buffer *t;
 
 	for (i = level; i < BTRFS_MAX_LEVEL; i++) {
@@ -1199,27 +1197,24 @@ static void unlock_up(struct btrfs_path *path, int level, int lowest_unlock)
 			break;
 		if (!path->locks[i])
 			break;
-		if (path->slots[i] == 0) {
+		if (!no_skips && path->slots[i] == 0) {
 			skip_level = i + 1;
 			continue;
 		}
-		if (path->keep_locks) {
+		if (!no_skips && path->keep_locks) {
 			u32 nritems;
 			t = path->nodes[i];
 			nritems = btrfs_header_nritems(t);
-			if (nritems < 2 || path->slots[i] >= nritems - 2) {
-if (path->keep_locks) {
-//printk("path %p skip level now %d\n", path, skip_level);
-}
+			if (nritems < 1 || path->slots[i] >= nritems - 1) {
 				skip_level = i + 1;
 				continue;
 			}
 		}
+		if (skip_level < i && i >= lowest_unlock)
+			no_skips = 1;
+
 		t = path->nodes[i];
 		if (i >= lowest_unlock && i > skip_level && path->locks[i]) {
-if (path->keep_locks) {
-//printk("path %p unlocking level %d slot %d nritems %d skip_level %d\n", path, i, path->slots[i], btrfs_header_nritems(t), skip_level);
-}
 			btrfs_tree_unlock(t);
 			path->locks[i] = 0;
 		}
@@ -1244,6 +1239,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root
 		      ins_len, int cow)
 {
 	struct extent_buffer *b;
+	struct extent_buffer *tmp;
 	int slot;
 	int ret;
 	int level;
@@ -1263,10 +1259,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root
 	if (ins_len < 0)
 		lowest_unlock = 2;
 again:
-	if (!p->skip_locking)
 	b = btrfs_lock_root_node(root);
-	else
-		b = btrfs_root_node(root);
 
 	while (b) {
 		level = btrfs_header_level(b);
@@ -1286,7 +1279,6 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root
 			WARN_ON(1);
 		level = btrfs_header_level(b);
 		p->nodes[level] = b;
-		if (!p->skip_locking)
 		p->locks[level] = 1;
 		ret = check_block(root, p, level);
 		if (ret)
@@ -1328,10 +1320,29 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root
 				reada_for_search(root, p, level, slot,
 						 key->objectid);
 
+			tmp = btrfs_find_tree_block(root,
+					  btrfs_node_blockptr(b, slot),
+					  btrfs_level_size(root, level - 1));
+			if (tmp && btrfs_buffer_uptodate(tmp,
+				   btrfs_node_ptr_generation(b, slot))) {
+				b = tmp;
+			} else {
+				/*
+				 * reduce lock contention at high levels
+				 * of the btree by dropping locks before
+				 * we read.
+				 */
+				if (level > 1) {
+					btrfs_release_path(NULL, p);
+					if (tmp)
+						free_extent_buffer(tmp);
+					goto again;
+				} else {
 					b = read_node_slot(root, b, slot);
-			if (!p->skip_locking)
+				}
+			}
 			btrfs_tree_lock(b);
-			unlock_up(p, level + 1, lowest_unlock);
+			unlock_up(p, level, lowest_unlock);
 		} else {
 			p->slots[level] = slot;
 			if (ins_len > 0 && btrfs_leaf_free_space(root, b) <
@@ -3007,17 +3018,8 @@ int btrfs_next_leaf(struct btrfs_root *root, struct btrfs_path *path)
 			reada_for_search(root, path, level, slot, 0);
 
 		next = read_node_slot(root, c, slot);
-		if (!path->skip_locking) {
-			if (!btrfs_tree_locked(c)) {
-				int i;
-				WARN_ON(1);
-printk("path %p no lock on level %d\n", path, level);
-for (i = 0; i < BTRFS_MAX_LEVEL; i++) {
-printk("path %p level %d slot %d nritems %d\n", path, i, path->slots[i], btrfs_header_nritems(path->nodes[i]));
-}
-			}
+		WARN_ON(!btrfs_tree_locked(c));
 		btrfs_tree_lock(next);
-		}
 		break;
 	}
 	path->slots[level] = slot;
@@ -3035,11 +3037,9 @@ printk("path %p level %d slot %d nritems %d\n", path, i, path->slots[i], btrfs_h
 		if (level == 1 && path->locks[1] && path->reada)
 			reada_for_search(root, path, level, slot, 0);
 		next = read_node_slot(root, next, 0);
-		if (!path->skip_locking) {
 		WARN_ON(!btrfs_tree_locked(path->nodes[level]));
 		btrfs_tree_lock(next);
 	}
-	}
 done:
 	unlock_up(path, 0, 1);
 	return 0;

fs/btrfs/ctree.h

@@ -336,7 +336,6 @@ struct btrfs_path {
 	/* keep some upper locks as we walk down */
 	int keep_locks;
 	int lowest_level;
-	int skip_locking;
 };
 
 /*

fs/btrfs/disk-io.c

@@ -1684,6 +1684,7 @@ void btrfs_throttle(struct btrfs_root *root)
 #else
 		blk_congestion_wait(WRITE, HZ/20);
 #endif
+
 	}
 }
 

fs/btrfs/extent-tree.c

@@ -88,7 +88,6 @@ static int cache_block_group(struct btrfs_root *root,
 		return -ENOMEM;
 
 	path->reada = 2;
-	path->skip_locking = 1;
 	first_free = block_group->key.objectid;
 	key.objectid = block_group->key.objectid;
 	key.offset = 0;