Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())

[Al Viro]

On Mon, Mar 12, 2018 at 03:23:44PM -0500, Eric W. Biederman wrote:

> Of the two code paths you are concert about:
> 
> For path path_connected looking at s_root is a heuristic to avoid
> calling is_subdir every time we need to do that check.  If the heuristic
> fails we still have is_subdir which should remain accurate.  If
> is_subdir fails the path is genuinely not connected at that moment
> and failing is the correct thing to do.
 
Umm...  That might be not good enough - the logics is "everything's
reachable from ->s_root anyway, so we might as well not bother checking".
For NFS it's simply not true.

We can mount server:/foo/bar/baz on /tmp/a, then server:/foo on /tmp/b
and we'll have ->s_root pointing to a subtree of what's reachable at
/tmp/b.  Play with renames under /tmp/b and you just might end up with
a problem.  And mount on /tmp/a will be (mistakenly) considered to
be safe, since it satisfies the heuristics in path_connected().