linux-kernel - Re: v4.2-rc dcache regression, probably 75a6f82a0d10

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Message-ID: <20150801170936.GX17109@ZenIV.linux.org.uk>
Date:	Sat, 1 Aug 2015 18:09:36 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dominique Martinet <asmadeus@...ewreck.org>,
	Hugh Dickins <hughd@...gle.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Dominique Martinet <dominique.martinet@....fr>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: v4.2-rc dcache regression, probably 75a6f82a0d10

On Sat, Aug 01, 2015 at 09:09:24AM -0700, Linus Torvalds wrote:
> On Sat, Aug 1, 2015 at 12:26 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > Actually, the shit had hit the fan earlier.  Look: in
> > commit b18825a7c8e37a7cf6abb97a12a6ad71af160de7
> > Author: David Howells <dhowells@...hat.com>
> > Date:   Thu Sep 12 19:22:53 2013 +0100
> >
> >     VFS: Put a small type field into struct dentry::d_flags
> >
> > we have this:
> >
> > -               if (!can_lookup(nd->inode)) {
> > +               if (!d_is_directory(nd->path.dentry)) {
> 
> Ahh. That's subtle, yes. Inodes are stable in ways that dentries
> aren't. And the reason why Dominique bisected it to 4bf46a272647 would
> seem to be that while dentries aren't really stable, the dentry flags
> generally don't change. But dentry_iput() changed to actually clear
> the type when clearing the inode, so that probably added a few cases
> where it went from "stable in practice" to be more easily triggered.
> 
> Your patch looks obviously correct, with the slight worry that there
> might be other cases of this.
> 
> > there.  AFAICS, other places of that sort are not a problem anymore.
> 
> Al, do you plan a pull request? It would be good to get this into rc5
> (tomorrow) regardless of whether there might be other issues lurking
> too.

Will do later today.  -stable branches will be interesting, though ;-/

Al, still digging through the loads of fun stuff in l-k mailbox...
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