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SubjectRe: Linux 2.6.25-rc2
Ingo Molnar wrote:
> * Pekka Enberg <penberg@cs.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@elte.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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