**Root Cause Analysis:**

The code snippet you provided contains a CWE-787 vulnerability (out-of-bounds write) at line 8 because the 'btrfs_double_lock' function is called without checking whether the 'len' parameter is greater than the size of the files being locked. If 'len' is greater than the file size, then 'btrfs_double_lock' may attempt to lock memory beyond the end of the files, leading to an out-of-bounds write.