commit b12cea9198fa99ffd3de1776c323bc7464d26b44
author Al Viro <viro@zeniv.linux.org.uk> Fri Mar 18 08:55:38 2011 -0400
committer Al Viro <viro@zeniv.linux.org.uk> Fri Mar 18 08:55:38 2011 -0400
tree 3ae5818b2690e45c8a71432ed681751683091287
parent 27cb1572e3e6bb1f8cf6bb3d74c914a87b131792

change the locking order for namespace_sem

Have it nested inside ->i_mutex.  Instead of using follow_down()
under namespace_sem, followed by grabbing i_mutex and checking that
mountpoint to be is not dead, do the following:
	grab i_mutex
	check that it's not dead
	grab namespace_sem
	see if anything is mounted there
	if not, we've won
	otherwise
		drop locks
		put_path on what we had
		replace with what's mounted
		retry everything with new mountpoint to be

New helper (lock_mount()) does that.  do_add_mount(), do_move_mount(),
do_loopback() and pivot_root() switched to it; in case of the last
two that eliminates a race we used to have - original code didn't
do follow_down().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>