linux-kernel - Re: vfs-scale, d_revalidate from nfsd

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Message-ID: <20110114030325.GU19804@ZenIV.linux.org.uk>
Date:	Fri, 14 Jan 2011 03:03:25 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Nick Piggin <npiggin@...il.com>
Cc:	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: vfs-scale, d_revalidate from nfsd

On Fri, Jan 14, 2011 at 01:57:55PM +1100, Nick Piggin wrote:
> On Fri, Jan 14, 2011 at 1:03 AM, J. R. Okajima <hooanon05@...oo.co.jp> wrote:
> >
> > NFSD calls filesystem's ->d_revalidate() with the parameter nd == NULL.
> > So every
> > ? ? ? ?if (nd->flags & LOOKUP_RCU)
> > ? ? ? ? ? ? ? ?return -ECHILD;
> > code which was added to ->d_revalidate() of FS which supports NFS
> > exporting will crash.
> >
> > If we rewrite it as
> > ? ? ? ?if (nd && (nd->flags & LOOKUP_RCU))
> > ? ? ? ? ? ? ? ?return -ECHILD;
> > the problem may not occur.
> > But I am not sure whether lookup_one_len() call in NFSD support rcu-walk.
> 
> Ah, good catch.
> 
> I'm going to change the d_revalidate API so it takes and inode and rcu-walk
> flag parameter to make it easier for filesystems to implement rcu-walk.
> 
> That will take care of this NULL nd case.

Oh, for fuck sake...  Would you at least mind posting your API change
description to fsdevel before going for it?

->d_revalidate() is one sick puppy and it's intimately involved in atomic
open rewrite.  Please, *please* don't make that shit even messier...
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