Re: [RFC PATCH] file as directory

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

On Wed, May 23, 2007 at 04:32:37PM +0200, Miklos Szeredi wrote:
> > Umm...  It is related to detached subtrees, but I'm not sure if it is what
> > you are thinking about.
> 
> I was thinking of a similar one by Mike Waychison.  It had the problem
> of requiring a spinlock for mntget/mntput.  It was also different in
> that it did not gradually dissolve detached trees, but kept them as
> whole blobs until the last ref went away.
 
Here the spinlock is needed only when mnt_busy goes to 0, so presumably
it won't be a serious problem on more or less common setups; however,
it certainly would need serious profiling.

> How will this work with copy_tree() and namespace duplication, which
> currently walk the tree with only namespace_sem held?

Easy - grab namespace_sem, grab vfsmount_lock, walk the subtree and bump
mnt_busy on everything (by 1 + number of non-busy children).  Then drop
vfsmount_lock and do as usual, dropping references in tree being copied
as you go.  Nothing will get attached or detached due to namespace_sem,
nothing will get evicted by anybody other than you since you've got all
that stuff pinned down.  End of story...