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The functions ovl_lower_positive() and ovl_check_empty_dir() both take
inode mutex on the real lower dir under ovl_want_write() which takes
the upper_mnt sb_writers lock.
While this is not a clear locking order or layering violation, it creates
an undesired lock dependency between two unrelated layers for no good
reason.
This lock dependency materializes to a false(?) positive lockdep warning
when calling rmdir() on a nested overlayfs, where both nested and
underlying overlayfs both use the same fs type as upper layer.
rmdir() on the nested overlayfs creates the lock chain:
sb_writers of upper_mnt (e.g. tmpfs) in ovl_do_remove()
ovl_i_mutex_dir_key[] of lower overlay dir in ovl_lower_positive()
rmdir() on the underlying overlayfs creates the lock chain in
reverse order:
ovl_i_mutex_dir_key[] of lower overlay dir in vfs_rmdir()
sb_writers of nested upper_mnt (e.g. tmpfs) in ovl_do_remove()
To rid of the unneeded locking dependency, move both ovl_lower_positive()
and ovl_check_empty_dir() to before ovl_want_write() in rmdir() and
rename() implementation.
This change spreads the pieces of ovl_check_empty_and_clear() directly
inside the rmdir()/rename() implementations so the helper is no longer
needed and removed.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
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