From: Andrey Borzenkov <arvidjaar@mail.ru>

I finally hit a painfully trivial way to reproduce another long standing devfs
problem - deadlock between devfs_lookup and devfs_d_revalidate_wait. When
devfs_lookup releases directory i_sem devfs_d_revalidate_wait grabs it (it
happens not for every path) and goes to wait to be waked up. Unfortunately,
devfs_lookup attempts to acquire directory i_sem before ever waking it up ...

To reproduce (2.5.74 UP or SMP - does not matter, single CPU system)

ls /dev/foo & rm -f /dev/foo &

or possibly in a loop but then it easily fills up process table. In my case it
hangs 100% reliably - on 2.5 OR 2.4.

The current fix is to move re-acquire of i_sem after all
devfs_d_revalidate_wait waiters have been waked up.  Much better fix would be
to ensure that ->d_revalidate either is always called under i_sem or always
without.  But that means the very heart of VFS and I do not dare to touch it.

The fix has been tested on 2.4 (and is part of unofficial Mandrake Club
kernel); I expected the same bug is in 2.5; I just was stupid not seeing the
way to reproduce it before.