Re: [patch 3/6] vfs: mountinfo stable peer group id

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Mar 22, 2008 at 03:49:50AM +0000, Al Viro wrote:

> 	Shifting increment from mnt_set_mountpoint() and commit_tree()
> to theirs callers and collapsing where possible, we get the following:
> 	* decrement in release_mounts() when resetting ->mnt_parent
> 	* increment in propagate_mnt() after call of mnt_set_mountpoint()
> 	* decrement in attach_recursive_mnt() in the loop calling
> commit_tree() for clones (on mountpoint of each clone).
> 	* increment in umount_tree() at the point where we update d_mounted.

... except that it'd give a leak in case of mount to shared mountpoint
failing halfway through - we'll get double increments since umount_tree()
would hit the mountpoints of cloned trees with extra increment, even though 
reference from root of cloned to its mountpoint is _already_ a ghost.

OTOH, we probably don't want to bother with counting those anyway - i.e.
it's simply a bad definition and the right one would be along the lines of
"number of vfsmounts that are doomed to be eaten by release_mounts() and
that have ->mnt_parent pointing to us".  IOW, dropping the 2nd and 3rd
in the above would do the right thing - anything chewed by umount_tree()
*will* go to release_mounts() and ones in flight are what we are interested
in...