linux-kernel - Re: [announce] vfs-scale git tree update

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Message-ID: <1294804776.2821.4.camel@perseus>
Date:	Wed, 12 Jan 2011 11:59:36 +0800
From:	Ian Kent <raven@...maw.net>
To:	aelder@....com
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [announce] vfs-scale git tree update

On Tue, 2011-01-11 at 11:57 -0600, Alex Elder wrote:
> On Tue, 2011-01-11 at 08:51 -0800, Linus Torvalds wrote:
> > On Tue, Jan 11, 2011 at 8:34 AM, Alex Elder <aelder@....com> wrote:
> > >
> > > FYI, when using this code, as merged by Linus, I hit the
> > > BUG_ON() at the beginning of d_set_d_op() when it's called
> > > by autofs4_dir_mkdir().  I managed to work around it by
> > > just commenting out those BUG_ON() calls but it's something
> > > that ought to get addressed properly.
> > 
> > Yeah, removing the BUG_ON() isn't the right thing to do - it means
> > that autofs4 is obviously setting the dentry ops twice for the same
> > dentry.
> > 
> > Possibly the thing could be relaxed to allow setting the _same_ d_op
> > pointer, ie do something like
> > 
> >    if (dentry->d_op == op)
> >       return;
> > 
> > at the top of that function. But looking at it, I don't think that
> > fixes the autofs4 issue.
> 
> That's easy enough, but it seems everybody else ensures
> this gets done just once per dentry, and it would be nice
> to preserve that "tightness" if possible.
> 
> > The fact that autofs4 does "d_add()" before it sets the d_ops (or
> > other dentry state, for that matter) looks a bit scary. To me that
> > smells like it might get a  dentry lookup hit before it's actually
> > fully done.
> 
> Agreed.

Isn't the parent i_mutex held during mkdir()?
Still the order can be changed, of course.

> 
> > Does it make any difference if you move the various d_add() calls down
> > to the end of the functions to when the "dentry" has really been
> > instantiated?
> 
> Looking at it quickly, I don't think that would matter for
> the case at hand.  I.e., that might be safer but it doesn't
> address the fact that these fields are getting initialized
> multiple times.

Yeah, a hangover from changes done over time.
Not setting the dentry op in ->lookup() should fix this.

> 
> 					-Alex
> 
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