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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Ingo Molnar <mingo@elte.hu>
Subject: Re: [bug] SLOB crash, 2.6.24-rc2
Date: Thu, 15 Nov 2007 21:57:59 +1100
User-Agent: KMail/1.9.5
Cc: David Miller <davem@davemloft.net>, mpm@selenic.com, rjw@sisk.pl,
       linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
       torvalds@linux-foundation.org
References: <20071114225335.GV19691@waste.org> <20071114.154143.112110604.davem@davemloft.net> <20071115104331.GA11390@elte.hu>
In-Reply-To: <20071115104331.GA11390@elte.hu>
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On Thursday 15 November 2007 21:43, Ingo Molnar wrote:
> * David Miller <davem@davemloft.net> wrote:
> > From: Matt Mackall <mpm@selenic.com>
> > Date: Wed, 14 Nov 2007 17:37:13 -0600
> >
> > > No, the usual strategy for debugging problems -outside- SLOB is to
> > > switch to another allocator with more extensive debugging facilities.
> >
> > Ok, so the thing we still can do is do a dump_stack() at the list
> > debugging assertion trigger points.
>
> ok, i'll first try to trigger it again.

I had implemented SLOB in userspace, so I resynched and think I
found your problem. Sorry for the attachment format -- this mailer
isn't the best. I'm really computer illiterate when it comes to
userspace...

Anyway, I'm really happy to see you're testing and using SLOB
upstream :) Is there any particular reason that you're using it?

Thanks,
Nick

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Previously, it would be possible for prev->next to point to &free_slob_pages,
and thus we would try to move a list onto itself, and bad things would
happen.

It seems a bit hairy to be doing list operations with the list marker as an
entry, rather than a head, but...

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/mm/slob.c
===================================================================
--- linux-2.6.orig/mm/slob.c
+++ linux-2.6/mm/slob.c
@@ -321,7 +321,8 @@ static void *slob_alloc(size_t size, gfp
 		/* Improve fragment distribution and reduce our average
 		 * search time by starting our next search here. (see
 		 * Knuth vol 1, sec 2.5, pg 449) */
-		if (free_slob_pages.next != prev->next)
+		if (prev != free_slob_pages.prev &&
+				free_slob_pages.next != prev->next)
 			list_move_tail(&free_slob_pages, prev->next);
 		break;
 	}

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