Re: Linux 2.6.25-rc2

[Pekka Enberg]

Ingo Molnar wrote:
> * Pekka Enberg <penberg@xxxxxxxxxxxxxx> 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@xxxxxxx>
>
> ---
>  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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/