In the Linux kernel, the following vulnerability has been resolved:

ext4: fix circular lock dependency in ext4_ext_migrate

Move iput(tmp_inode) after ext4_writepages_up_write() to avoid a
circular lock dependency between s_writepages_rwsem and sb_internal
(freeze protection).

The deadlock scenario:

  CPU0 (EXT4_IOC_MIGRATE)        CPU1 (orphan cleanup during mount)
  ----                           ----
  ext4_ext_migrate()
    ext4_writepages_down_write()
      s_writepages_rwsem (write)
                                 ext4_evict_inode()
                                   sb_start_intwrite()   [sb_internal]
                                   ...
                                     ext4_writepages()
                                       s_writepages_rwsem (read) [BLOCKED]
    iput(tmp_inode)
      ext4_evict_inode()
        sb_start_intwrite()         [BLOCKED]

The tmp_inode is a temporary inode with nlink=0 created solely for
building the extent tree.  Its eviction does not require
s_writepages_rwsem protection, so deferring iput() until after
releasing the rwsem is safe.