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SubjectRe: [patch 00/13] vfs: add helpers to check r/o bind mounts
In message <20080424173013.GA5882@ZenIV.linux.org.uk>, Al Viro writes:
> On Thu, Apr 24, 2008 at 01:25:58PM -0400, Erez Zadok wrote:
> > Al, any near-term plans for sb-level "want write" locking as we discussed
> > briefly at LSF? Being able to do so for copyup in unionfs will hopefully
> > allow me to prevent concurrent topology changes.
>
> That's pretty close to the top.
>
> > the two directories that need locking. This rename-locking protocol appears
> > to be a special case of the sb-level "want write" idea, right?
>
> Not really - that one doesn't provide any exclusion between writers and
> that's what we want from rename/rename.

When you say "that one", do you mean the sb-level idea? If the sb-level
"want write" not provide exclusion among writers, then how can I prevent
lower topology changes while copyup takes place?

If your sb-level "want write" idea won't provide that exclusion at the sb
level, then how about we elevate the s_vfs_rename_mutex into a generic
s_vfs_multi_dir_change_mutex of sorts, which can be used by rename as well
as copyup?

> Take a look at Documentation/filesystems/directory-locking for details
> of locking scheme...

OK. lock_rename() at most locks one "parent" and one "child" inodes. So
what happens to all the ones in b/t? Suppose I have a path /a/b/c/d/e/f and
someone wants to mv /a/b/c/d/e to /a/b/; in that case lock_rename at least
grabs the mutex on /a/b, right? Can someone go in the middle and try to
muck with the "c/d/" parts in between?

From what I gathered, lock_rename won't be enough for me to prevent topology
changes during copyup; I'd really need to lock an entire arbitrarily deep
path of inodes. That just seems so complex and prone to bugs, that it might
just be easier to have a single sb-level mutex for these complex
multi-directory operations. I'm not sure how much performance hit this'd
be, thou, and if there's a more efficient way to do so.

If you think lock_rename will be sufficient for me to deal with copyups
vs. topology changes, then I'll start using it.

Thanks,
Erez.


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