linux-kernel - Re: [Bugme-new] [Bug 7495] New: Kernel periodically hangs.

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Message-ID: <17752.5086.510190.316725@cse.unsw.edu.au>
Date:	Mon, 13 Nov 2006 17:42:38 +1100
From:	Neil Brown <neilb@...e.de>
To:	Andrew Morton <akpm@...l.org>
Cc:	David Howells <dhowells@...hat.com>,
	"bugme-daemon@...nel-bugs.osdl.org" 
	<bugme-daemon@...zilla.kernel.org>, linux-kernel@...r.kernel.org,
	alex@...snet.ru
Subject: Re: [Bugme-new] [Bug 7495] New: Kernel periodically hangs.

On Saturday November 11, akpm@...l.org wrote:
> On Sat, 11 Nov 2006 03:29:32 -0800
> bugme-daemon@...zilla.kernel.org wrote:
> 
> > http://bugzilla.kernel.org/show_bug.cgi?id=7495
> > 
> >            Summary: Kernel periodically hangs.
> >     Kernel Version: Linux version 2.6.18.2 (root@pub) (gcc version 3.4.6)
> >                     #13 SMP Fr
> >             Status: NEW
> >           Severity: blocking
> >              Owner: other_other@...nel-bugs.osdl.org
> >          Submitter: alex@...snet.ru

So getting back to the main issue in this bug report.....

> > 
> > 
> > [42587.676000] BUG: unable to handle kernel NULL pointer dereference at 
> > virtual address 0000003c

it would appear that in:
	if (inode->i_sb && inode->i_sb->s_op->clear_inode)
		inode->i_sb->s_op->clear_inode(inode);

inode->i_sb->s_op is NULL.  This is unfortunate :-)
alloc_super initialises s_op to '&default_op' and it isn't cleared on
unmount, so the implication seems to be that i_sb has been freed and
the memory has been reused.  This tends to suggest that
generic_shutdown_super isn't releasing all inodes before the
superblock gets destroyed.

I cannot see how this could be happening yet, but it might be helpful
to compile with CONFIG_DEBUG_SLAB and maybe even
CONFIG_DEBUG_PAGEALLOC.
That might make the problem trigger earlier and so be easier to track.

NeilBrown
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