linux-kernel - Re: BUG_ON(nd->inode != parent->d

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Message-ID: <CA+55aFxKgZyqbn1M0sAL0CoLxmKSzAALYBTPLYVM7OpG_M_B=A@mail.gmail.com>
Date:	Fri, 8 Mar 2013 15:20:40 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: BUG_ON(nd->inode != parent->d_inode);

On Fri, Mar 8, 2013 at 3:07 PM, Dave Jones <davej@...hat.com> wrote:
>
> Ok, got something more meaningful out of the lookup_slow trace.
>
> [   66.082984] parent->dname.name  (06b6b6b6b6b6b6b)
> [   66.083637] parent =
>
> At first I thought AH-HA! SLAB POISON!
> But look closer.. it's shifted by 8 bits.

Or just the high byte has been cleared.

But yeah, if the parent has been free'd then that certainly explains
why the "impossible" test of

  nd->inode != parent->d_inode

would trigger. And it would explain any odd crashes at lookup time
too. In particular, the NULL pointer one you reference seems to be
dir->i_op->lookup being NULL, so calling it (understandable) ends up
doing bad things.

I really don't understand how the parent could be free'd early.
Dentries are freed by RCU, and the dentry lookup code is some of the
most well-tested out there. I don't see how /proc could mess that up,
unless it just completely screws up some refcounting thing or other.

                 Linus
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