from freebsd r1.185 (tested by beck@ and krw@; pedro@ ok): After a rmdir()ed directory has been truncated, force an update of the directory's inode after queuing the dirrem that will decrement the parent directory's link count.  This will force the update of the parent directory's actual link to actually be scheduled.  Without this change the parent directory's actual link count would not be updated until ufs_inactive() cleared the inode of the newly removed directory, which might be deferred indefinitely.  ufs_inactive() will not be called as long as any process holds a reference to the removed directory, and ufs_inactive() will not clear the inode if the link count is non-zero, which could be the result of an earlier system crash.
from freebsd r1.185 (tested by beck@ and krw@; pedro@ ok):
After a rmdir()ed directory has been truncated, force an update of
the directory's inode after queuing the dirrem that will decrement
the parent directory's link count.  This will force the update of
the parent directory's actual link to actually be scheduled.  Without
this change the parent directory's actual link count would not be
updated until ufs_inactive() cleared the inode of the newly removed
directory, which might be deferred indefinitely.  ufs_inactive()
will not be called as long as any process holds a reference to the
removed directory, and ufs_inactive() will not clear the inode if
the link count is non-zero, which could be the result of an earlier
system crash.

If a background fsck is run before the update of the parent directory's
actual link count has been performed, or at least scheduled by
putting the dirrem on the leaf directory's inodedep id_bufwait list,
fsck will corrupt the file system by decrementing the parent
directory's effective link count, which was previously correct
because it already took the removal of the leaf directory into
account, and setting the actual link count to the same value as the
effective link count after the dangling, removed, leaf directory
has been removed.  This happens because fsck acts based on the
actual link count, which will be too high when fsck creates the
file system snapshot that it references.

This change has the fortunate side effect of more quickly cleaning
up the large number dirrem structures that linger for an extended
time after the removal of a large directory tree.  It also fixes a
potential problem with the shutdown of the syncer thread timing out
if the system is rebooted immediately after removing a large directory
tree.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars