linux-kernel - Re: Linux 2.6.25-rc2

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Message-ID: <64bb37e0802191127k2a202f4fm4bed2ea61c0fd7a8@mail.gmail.com>
Date:	Tue, 19 Feb 2008 20:27:47 +0100
From:	"Torsten Kaiser" <just.for.lkml@...glemail.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Pekka Enberg" <penberg@...helsinki.fi>,
	"Mathieu Desnoyers" <mathieu.desnoyers@...ymtl.ca>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Christoph Lameter" <clameter@....com>
Subject: Re: Linux 2.6.25-rc2

On Feb 19, 2008 5:20 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> So:
>  - it might be something else entirely
>  - it might still be the local cmpxchg, just Torsten didn't happen to
>    notice it until later.

My new hackbench-testcase also killed 2.6.24-rc2-mm1, so I really
noticed to late.

>  - it might still be the local cmpxchg, but something else changed its
>    patterns to actually make it start triggering.
>
> and in general I don't think we should revert it unless we have stronger
> indications that it really is the problem (eg somebody finds the actual
> bug, or a reporter can confirm that it goes away when the local cmpxchg
> optimization is disabled).

I tried the following three patches:

switching the barrier() for a smp_mb() in 2.6.25-rc2-mm1:
-> crashed

reverting the FASTPATH-patch in 2.6.25-rc2:
-> worked

only removed FAST_CMPXCHG_LOCAL from arch/x86/Kconfig
-> worked

So all of these tests seem to confirm, that the bug is in the new SLUB fastpath.

Torsten
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