linux-kernel - Re: Linux 2.6.25-rc2

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Date:	Tue, 19 Feb 2008 12:45:29 +0200
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Torsten Kaiser" <just.for.lkml@...glemail.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Mathieu Desnoyers" <mathieu.desnoyers@...ymtl.ca>,
	"Christoph Lameter" <clameter@....com>
Subject: Re: Linux 2.6.25-rc2

Hi,

Pekka Enberg <penberg@...helsinki.fi> wrote:
> > Mathieu, Christoph is on vacation and I'm not at all that familiar
> > with this cmpxchg_local() optimization, so if you could take a peek at
> > this bug report to see if you can spot something obviously wrong with
> > it, I would much appreciate that.

On Feb 19, 2008 12:27 PM, Ingo Molnar <mingo@...e.hu> wrote:
> hm, it's bad for at least one other reason as well (which is probably
> unrelated to this crash):
>
>  /*
>   * Currently fastpath is not supported if preemption is enabled.
>   */
>  #if defined(CONFIG_FAST_CMPXCHG_LOCAL) && !defined(CONFIG_PREEMPT)
>  #define SLUB_FASTPATH
>  #endif
>
> such !PREEMPT exceptions tend to show "i didnt want to think too hard
> about the preemptible case so just turn it off" thinking.
>
> Also, why isnt this "SLUB_FASTPATH" flag done in the Kconfig space?

Hmm, no idea. I think might have been some mix-up with merging the
patch. The one I saw was:

http://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.24/2.6.24-mm1/broken-out/slub-optional-fast-path-using-cmpxchg_local.patch

But I don't remember giving out a Reviewed-by for it (and my mailbox
confirms that). Furthermore, somehow it turned into this when merged:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=1f84260c8ce3b1ce26d4c1d6dedc2f33a3a29c0c

In any case, if Torsten/someone can verify that reverting
1f84260c8ce3b1ce26d4c1d6dedc2f33a3a29c0c ("SLUB: Alternate fast paths
using cmpxchg_local") fixes these problems, I think we should just do
it and let Christoph sort it out when he gets back.

                               Pekka
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