Btrfs: fix deadlock on tree root leaf when finding free extent

When we are writing out a free space cache, during the transaction commit
phase, we can end up in a deadlock which results in a stack trace like the
following:

schedule+0x28/0x80
btrfs_tree_read_lock+0x8e/0x120 [btrfs]
? finish_wait+0x80/0x80
btrfs_read_lock_root_node+0x2f/0x40 [btrfs]
btrfs_search_slot+0xf6/0x9f0 [btrfs]
? evict_refill_and_join+0xd0/0xd0 [btrfs]
? inode_insert5+0x119/0x190
btrfs_lookup_inode+0x3a/0xc0 [btrfs]
? kmem_cache_alloc+0x166/0x1d0
btrfs_iget+0x113/0x690 [btrfs]
__lookup_free_space_inode+0xd8/0x150 [btrfs]
lookup_free_space_inode+0x5b/0xb0 [btrfs]
load_free_space_cache+0x7c/0x170 [btrfs]
? cache_block_group+0x72/0x3b0 [btrfs]
cache_block_group+0x1b3/0x3b0 [btrfs]
? finish_wait+0x80/0x80
find_free_extent+0x799/0x1010 [btrfs]
btrfs_reserve_extent+0x9b/0x180 [btrfs]
btrfs_alloc_tree_block+0x1b3/0x4f0 [btrfs]
__btrfs_cow_block+0x11d/0x500 [btrfs]
btrfs_cow_block+0xdc/0x180 [btrfs]
btrfs_search_slot+0x3bd/0x9f0 [btrfs]
btrfs_lookup_inode+0x3a/0xc0 [btrfs]
? kmem_cache_alloc+0x166/0x1d0
btrfs_update_inode_item+0x46/0x100 [btrfs]
cache_save_setup+0xe4/0x3a0 [btrfs]
btrfs_start_dirty_block_groups+0x1be/0x480 [btrfs]
btrfs_commit_transaction+0xcb/0x8b0 [btrfs]

At cache_save_setup() we need to update the inode item of a block group's
cache which is located in the tree root (fs_info->tree_root), which means
that it may result in COWing a leaf from that tree. If that happens we
need to find a free metadata extent and while looking for one, if we find
a block group which was not cached yet we attempt to load its cache by
calling cache_block_group(). However this function will try to load the
inode of the free space cache, which requires finding the matching inode
item in the tree root - if that inode item is located in the same leaf as
the inode item of the space cache we are updating at cache_save_setup(),
we end up in a deadlock, since we try to obtain a read lock on the same
extent buffer that we previously write locked.

So fix this by using the tree root's commit root when searching for a
block group's free space cache inode item when we are attempting to load
a free space cache. This is safe since block groups once loaded stay in
memory forever, as well as their caches, so after they are first loaded
we will never need to read their inode items again. For new block groups,
once they are created they get their ->cached field set to
BTRFS_CACHE_FINISHED meaning we will not need to read their inode item.

Reported-by: Andrew Nelson <[e-mail]>
Link: https://lore.kernel.org/linux-btrfs/[e-mail]/
Fixes: 9d66e233c704 ("Btrfs: load free space cache if it exists")
Tested-by: Andrew Nelson <[e-mail]>
Reviewed-by: Josef Bacik <[e-mail]>
Signed-off-by: Filipe Manana <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/ctree.h

···
 int btrfs_drop_inode(struct inode *inode);
 int __init btrfs_init_cachep(void);
 void __cold btrfs_destroy_cachep(void);
+struct inode *btrfs_iget_path(struct super_block *s, struct btrfs_key *location,
+			      struct btrfs_root *root, int *new,
+			      struct btrfs_path *path);
 struct inode *btrfs_iget(struct super_block *s, struct btrfs_key *location,
 			 struct btrfs_root *root, int *was_new);
 struct extent_map *btrfs_get_extent(struct btrfs_inode *inode,

fs/btrfs/free-space-cache.c

···
 	 * sure NOFS is set to keep us from deadlocking.
 	 */
 	nofs_flag = memalloc_nofs_save();
-	inode = btrfs_iget(fs_info->sb, &location, root, NULL);
+	inode = btrfs_iget_path(fs_info->sb, &location, root, NULL, path);
+	btrfs_release_path(path);
 	memalloc_nofs_restore(nofs_flag);
 	if (IS_ERR(inode))
 		return inode;
···
 	path->search_commit_root = 1;
 	path->skip_locking = 1;
 
+	/*
+	 * We must pass a path with search_commit_root set to btrfs_iget in
+	 * order to avoid a deadlock when allocating extents for the tree root.
+	 *
+	 * When we are COWing an extent buffer from the tree root, when looking
+	 * for a free extent, at extent-tree.c:find_free_extent(), we can find
+	 * block group without its free space cache loaded. When we find one
+	 * we must load its space cache which requires reading its free space
+	 * cache's inode item from the root tree. If this inode item is located
+	 * in the same leaf that we started COWing before, then we end up in
+	 * deadlock on the extent buffer (trying to read lock it when we
+	 * previously write locked it).
+	 *
+	 * It's safe to read the inode item using the commit root because
+	 * block groups, once loaded, stay in memory forever (until they are
+	 * removed) as well as their space caches once loaded. New block groups
+	 * once created get their ->cached field set to BTRFS_CACHE_FINISHED so
+	 * we will never try to read their inode item while the fs is mounted.
+	 */
 	inode = lookup_free_space_inode(fs_info, block_group, path);
 	if (IS_ERR(inode)) {
 		btrfs_free_path(path);

fs/btrfs/inode.c

···
 /*
  * read an inode from the btree into the in-memory inode
  */
-static int btrfs_read_locked_inode(struct inode *inode)
+static int btrfs_read_locked_inode(struct inode *inode,
+				   struct btrfs_path *in_path)
 {
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
-	struct btrfs_path *path;
+	struct btrfs_path *path = in_path;
 	struct extent_buffer *leaf;
 	struct btrfs_inode_item *inode_item;
 	struct btrfs_root *root = BTRFS_I(inode)->root;
···
 	if (!ret)
 		filled = true;
 
-	path = btrfs_alloc_path();
-	if (!path)
-		return -ENOMEM;
+	if (!path) {
+		path = btrfs_alloc_path();
+		if (!path)
+			return -ENOMEM;
+	}
 
 	memcpy(&location, &BTRFS_I(inode)->location, sizeof(location));
 
 	ret = btrfs_lookup_inode(NULL, root, path, &location, 0);
 	if (ret) {
-		btrfs_free_path(path);
+		if (path != in_path)
+			btrfs_free_path(path);
 		return ret;
 	}
 
···
 				  btrfs_ino(BTRFS_I(inode)),
 				  root->root_key.objectid, ret);
 	}
-	btrfs_free_path(path);
+	if (path != in_path)
+		btrfs_free_path(path);
 
 	if (!maybe_acls)
 		cache_no_acl(inode);
···
 /* Get an inode object given its location and corresponding root.
  * Returns in *is_new if the inode was read from disk
  */
-struct inode *btrfs_iget(struct super_block *s, struct btrfs_key *location,
-			 struct btrfs_root *root, int *new)
+struct inode *btrfs_iget_path(struct super_block *s, struct btrfs_key *location,
+			      struct btrfs_root *root, int *new,
+			      struct btrfs_path *path)
 {
 	struct inode *inode;
 
···
 	if (inode->i_state & I_NEW) {
 		int ret;
 
-		ret = btrfs_read_locked_inode(inode);
+		ret = btrfs_read_locked_inode(inode, path);
 		if (!ret) {
 			inode_tree_add(inode);
 			unlock_new_inode(inode);
···
 	}
 
 	return inode;
+}
+
+struct inode *btrfs_iget(struct super_block *s, struct btrfs_key *location,
+			 struct btrfs_root *root, int *new)
+{
+	return btrfs_iget_path(s, location, root, new, NULL);
 }
 
 static struct inode *new_simple_dir(struct super_block *s,