Btrfs: Change find_extent_buffer to use TestSetPageLocked

This makes it possible for callers to check for extent_buffers in cache
without deadlocking against any btree locks held.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -1438,7 +1438,6 @@ static int pin_down_bytes(struct btrfs_root *root, u64 bytenr, u32 num_bytes,
 	int err = 0;
 
 	if (!pending) {
-#if 0
 		struct extent_buffer *buf;
 		buf = btrfs_find_tree_block(root, bytenr, num_bytes);
 		if (buf) {
@@ -1460,7 +1459,6 @@ static int pin_down_bytes(struct btrfs_root *root, u64 bytenr, u32 num_bytes,
 			}
 			free_extent_buffer(buf);
 		}
-#endif
 		update_pinned_extents(root, bytenr, num_bytes, 1);
 	} else {
 		set_extent_bits(&root->fs_info->pending_del,

fs/btrfs/extent_io.c

@@ -2808,10 +2808,15 @@ struct extent_buffer *find_extent_buffer(struct extent_io_tree *tree,
 		goto lru_add;
 
 	for (i = 0; i < num_pages; i++, index++) {
-		p = find_lock_page(mapping, index);
+		p = find_get_page(mapping, index);
 		if (!p) {
 			goto fail;
 		}
+		if (TestSetPageLocked(p)) {
+			page_cache_release(p);
+			goto fail;
+		}
+
 		set_page_extent_mapped(p);
 		mark_page_accessed(p);