Btrfs: Fix snapshot deletion to release the alloc_mutex much more often.

This lowers the impact of snapshot deletion on the rest of the FS.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.c

@@ -1255,7 +1255,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root
 	lowest_level = p->lowest_level;
 	WARN_ON(lowest_level && ins_len);
 	WARN_ON(p->nodes[0] != NULL);
-	WARN_ON(root == root->fs_info->extent_root &&
+	WARN_ON(cow && root == root->fs_info->extent_root &&
 		!mutex_is_locked(&root->fs_info->alloc_mutex));
 	WARN_ON(root == root->fs_info->chunk_root &&
 		!mutex_is_locked(&root->fs_info->chunk_mutex));

fs/btrfs/disk-io.c

@@ -1674,6 +1674,7 @@ void btrfs_mark_buffer_dirty(struct extent_buffer *buf)
 
 void btrfs_throttle(struct btrfs_root *root)
 {
+#if 0
 	struct backing_dev_info *bdi;
 
 	bdi = &root->fs_info->bdi;
@@ -1686,6 +1687,7 @@ void btrfs_throttle(struct btrfs_root *root)
 #endif
 
 	}
+#endif
 }
 
 void btrfs_btree_balance_dirty(struct btrfs_root *root, unsigned long nr)

fs/btrfs/extent-tree.c

@@ -1223,8 +1223,8 @@ printk("space info full %Lu\n", flags);
 	ret = btrfs_make_block_group(trans, extent_root, 0, flags,
 		     BTRFS_FIRST_CHUNK_TREE_OBJECTID, start, num_bytes);
 	BUG_ON(ret);
-	mutex_unlock(&extent_root->fs_info->chunk_mutex);
 out:
+	mutex_unlock(&extent_root->fs_info->chunk_mutex);
 	return 0;
 }
 
@@ -2181,17 +2181,29 @@ static void noinline reada_walk_down(struct btrfs_root *root,
 				continue;
 			}
 		}
-		mutex_unlock(&root->fs_info->alloc_mutex);
 		ret = readahead_tree_block(root, bytenr, blocksize,
 					   btrfs_node_ptr_generation(node, i));
 		last = bytenr + blocksize;
 		cond_resched();
-		mutex_lock(&root->fs_info->alloc_mutex);
 		if (ret)
 			break;
 	}
 }
 
+/*
+ * we want to avoid as much random IO as we can with the alloc mutex
+ * held, so drop the lock and do the lookup, then do it again with the
+ * lock held.
+ */
+int drop_snap_lookup_refcount(struct btrfs_root *root, u64 start, u64 len,
+			      u32 *refs)
+{
+	mutex_unlock(&root->fs_info->alloc_mutex);
+	lookup_extent_ref(NULL, root, start, len, refs);
+	mutex_lock(&root->fs_info->alloc_mutex);
+	return lookup_extent_ref(NULL, root, start, len, refs);
+}
+
 /*
  * helper function for drop_snapshot, this walks down the tree dropping ref
  * counts as it goes.
@@ -2215,8 +2227,7 @@ static int noinline walk_down_tree(struct btrfs_trans_handle *trans,
 
 	WARN_ON(*level < 0);
 	WARN_ON(*level >= BTRFS_MAX_LEVEL);
-	ret = lookup_extent_ref(trans, root,
-				path->nodes[*level]->start,
+	ret = drop_snap_lookup_refcount(root, path->nodes[*level]->start,
 				path->nodes[*level]->len, &refs);
 	BUG_ON(ret);
 	if (refs > 1)
@@ -2245,7 +2256,7 @@ static int noinline walk_down_tree(struct btrfs_trans_handle *trans,
 		ptr_gen = btrfs_node_ptr_generation(cur, path->slots[*level]);
 		blocksize = btrfs_level_size(root, *level - 1);
 
-		ret = lookup_extent_ref(trans, root, bytenr, blocksize, &refs);
+		ret = drop_snap_lookup_refcount(root, bytenr, blocksize, &refs);
 		BUG_ON(ret);
 		if (refs != 1) {
 			parent = path->nodes[*level];
@@ -2261,15 +2272,16 @@ static int noinline walk_down_tree(struct btrfs_trans_handle *trans,
 		next = btrfs_find_tree_block(root, bytenr, blocksize);
 		if (!next || !btrfs_buffer_uptodate(next, ptr_gen)) {
 			free_extent_buffer(next);
+			mutex_unlock(&root->fs_info->alloc_mutex);
+
 			reada_walk_down(root, cur, path->slots[*level]);
 
-			mutex_unlock(&root->fs_info->alloc_mutex);
 			next = read_tree_block(root, bytenr, blocksize,
 					       ptr_gen);
 			mutex_lock(&root->fs_info->alloc_mutex);
 
 			/* we've dropped the lock, double check */
-			ret = lookup_extent_ref(trans, root, bytenr,
+			ret = drop_snap_lookup_refcount(root, bytenr,
 						blocksize, &refs);
 			BUG_ON(ret);
 			if (refs != 1) {