Re: vfs-scale, general questions (Re: NFS root lockups with -next 20110113)

[Ian Kent <raven@themaw.net>]

On Wed, 2011-01-19 at 15:43 +0900, J. R. Okajima wrote:
> Hi,
> 
> Nick Piggin:
> > Thanks for your help, can you see how I've fixed it in my vfs-scale
> > tree? What do you think?
> 
> Your fix is great. I have no objection at all.
> Other than the fix, here are more generic questions about vfs-scale work.
> I am happy if you reply when you have time.
> 
> - getcwd(2) needs d_lock?
>   It acquires rename_lock and then tests whether the pwd is removed by
>   d_unhashed(). If a race condition between vfs_rename_dir() which may
>   unhash/rehash the dentry happens, then getcwd() may return the wrong
>   result due to unprotected d_unhashed() call, I am afraid. rename_lock
>   doesn't help this case.
> 
> - what is the right order of dget() and mntget()?
>   If I remember correctly, someone said "mntget() first and then
>   dget(). when putting, do in reverse" in the discussion when
>   path_{get,put}() were born. So it is called "the right order" in the
>   commit log.
>   It was many years ago. Is it still true? And should rcu-walk follow it
>   too? The current implementation doesn't seem to care about this order.

I didn't spot that, where did you see this?

I'm not sure about the get but I fairly sure the dput() has to be before
the mntput() because the shrink_dcache_*() cleanup routines object to
dentrys that have a reference count of more than one.

Ian