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

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Date:	Wed, 12 Jan 2011 14:55:20 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	aelder@....com, Ian Kent <raven@...maw.net>,
	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 Wed, Jan 12, 2011 at 5:13 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> [ Added Ian to cc - the BUG_ON() in d_set_d_op() triggers on autofs4,
> see lkml thread for more details if you care ]
>
> On Tue, Jan 11, 2011 at 9:57 AM, Alex Elder <aelder@....com> wrote:
>>
>> 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.
>
> You're right - without looking closer or thinking about it any more, I
> was thinking mkdir etc would create the new dentry, but it can easily
> be a pre-existing negative one. So it's not like mkdir is always going
> to be something new that can just be initialized.
>
> So yeah, no race or anything like that required - just regular acceses.
>
> That said, if we make sure that the d_op gets cleared (or the dentry
> dropped) when something turns negative, and a negative lookup never
> sets d_op, it should all basically work.
>
> Can you figure out the exact sequence that causes this? Is it enough
> to just mount something once?

Doh, sorry I probably didn't re-test autofs4 since putting that BUG_ON
in there. Now it's trivial to revert the d_set_d_op function to the same
raciness that was "allowed" before (simply by resetting the d_flags bits).
There is no new fundamental race that the patch introduced there...
But I think it is much cleaner to just fix up all filesystems, and will
potentially close obscure races.

Often the best way is to just set the same d_ops for negative and
positive dentries, and have the operations handle both cases.

Ian?
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