[bug report] btrfs: locking: Add extra check in btrfs_init_new_buffer() to avoid deadlock

[Dan Carpenter <dan.carpenter@oracle.com>]

[ This one is not actually your fault.  It's the core code which is
  confusing.  -dan ]

Hello Qu Wenruo,

The patch b72c3aba09a5: "btrfs: locking: Add extra check in
btrfs_init_new_buffer() to avoid deadlock" from Aug 21, 2018, leads
to the following static checker warning:

	fs/btrfs/extent-tree.c:8556 btrfs_init_new_buffer()
	warn: possible NULL dereference of 'buf'

fs/btrfs/extent-tree.c
    8540 static struct extent_buffer *
    8541 btrfs_init_new_buffer(struct btrfs_trans_handle *trans, struct btrfs_root *root,
    8542 		      u64 bytenr, int level, u64 owner)
    8543 {
    8544 	struct btrfs_fs_info *fs_info = root->fs_info;
    8545 	struct extent_buffer *buf;
    8546 
    8547 	buf = btrfs_find_create_tree_block(fs_info, bytenr);

The btrfs_find_create_tree_block() function either returns
alloc_test_extent_buffer() which returns NULL on error or
alloc_extent_buffer() which returns error pointers on error.  It
confuses static checkers and me also.

    8548 	if (IS_ERR(buf))
    8549 		return buf;
    8550 
    8551 	/*
    8552 	 * Extra safety check in case the extent tree is corrupted and extent
    8553 	 * allocator chooses to use a tree block which is already used and
    8554 	 * locked.
    8555 	 */
--> 8556 	if (buf->lock_owner == current->pid) {
    8557 		btrfs_err_rl(fs_info,
    8558 "tree block %llu owner %llu already locked by pid=%d, extent tree corruption detected",
    8559 			buf->start, btrfs_header_owner(buf), current->pid);
    8560 		free_extent_buffer(buf);

regards,
dan carpenter