linux-kernel - Re: Linux 2.6.25-rc2

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Date:	Tue, 19 Feb 2008 17:52:34 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Torsten Kaiser <just.for.lkml@...glemail.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Lameter <clameter@....com>,
	yanmin_zhang@...ux.intel.com
Subject: Re: Linux 2.6.25-rc2

Ingo Molnar wrote:
> * Pekka Enberg <penberg@...helsinki.fi> wrote:
> 
>>> Yes, this can happen. Are you saying it is not safe to be in the 
>>> lockless path when an IRQ triggers?
>> Hmm. The barrier() in slab_free() looks fishy. The comment says it's 
>> there to make sure we've retrieved c->freelist before c->page but then 
>> it uses a _compiler barrier_ which doesn't affect the CPU and the 
>> reads may still be re-ordered... Not sure if that matters here though.
> 
> find a fix patch for that below - most systems affected seem to be SMP 
> ones.
> 
> If this (or my other patch) indeed solves the problem i'd still favor a 
> full revert of the SLUB_FASTPATH (commit 1f84260c8ce3b1ce26d4), it looks 
> quite un-cooked and quite un-tested for multiple independent reasons.
> 
> Sigh, why do i again have to be the messenger who brings the bad news to 
> SLUB land, and again when poor Christoph went on vacation? :-/
> 
> 	Ingo
> 
> -------------------------->
> Subject: SLUB: barrier fix
> From: Ingo Molnar <mingo@...e.hu>
> 
> ---
>  mm/slub.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux/mm/slub.c
> ===================================================================
> --- linux.orig/mm/slub.c
> +++ linux/mm/slub.c
> @@ -1862,7 +1862,7 @@ static __always_inline void slab_free(st
>  	debug_check_no_locks_freed(object, s->objsize);
>  	do {
>  		freelist = c->freelist;
> -		barrier();
> +		smp_mb();
>  		/*
>  		 * If the compiler would reorder the retrieval of c->page to
>  		 * come before c->freelist then an interrupt could

Torsten/Yamin, does this fix things for you? What about reverting commit 
1f84260c8ce3b1ce26d4c1d6dedc2f33a3a29c0c ("SLUB: Alternate fast paths 
using cmpxchg_local")?

			Pekka
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