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

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Date:	Tue, 11 Jan 2011 11:57:48 -0600
From:	Alex Elder <aelder@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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 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.

> 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.

					-Alex

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