Re: [PATCH v3 1/1] dcache: Translating dentry into pathname without taking rename_lock

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

On Fri, Sep 06, 2013 at 02:48:32PM -0700, Linus Torvalds wrote:
> On Fri, Sep 6, 2013 at 2:05 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > I can take that, but I'm really not convinced that we need writer lock
> > there at all.  After all, if we really can get livelocks on that one,
> > we would be getting them on d_lookup()...
> 
> d_lookup() does a _single_ path component. That's a *big* difference.
> 
> Sure, the hash chain that d_lookup() (well, __d_lookup()) ends up
> walking is a bit more complicated than just following the dentry
> parent pointer, but that's much harder to trigger than just creating a
> really deep directory structure of single-letter nested directories,
> and then doing a "getcwd()" that walks 1024+ parents, while another
> thread is looping renaming things..
> 
> So I personally do feel a lot safer with the fallback to write locking here.
> 
> Especially since it's pretty simple, so there isn't really much downside.

Er... what will happen if you have done just what you've described and have
a process call d_lookup()?