Re: [PATCH 1/2] mm/slub: optimize alloc/free fastpath by removing preemption on/off

[Davidlohr Bueso]

On Mon, 2015-01-05 at 10:36 +0900, Joonsoo Kim wrote:
> -	preempt_disable();
> -	c = this_cpu_ptr(s->cpu_slab);
> +	do {
> +		tid = this_cpu_read(s->cpu_slab->tid);
> +		c = this_cpu_ptr(s->cpu_slab);
> +	} while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
> +	barrier();

I don't see the compiler reodering the object/page stores below, since c
is updated in the loop anyway. Is this really necessary (same goes for
slab_free)? The generated code by gcc 4.8 looks correct without it.
Additionally, the implied barriers for preemption control aren't really
the same semantics used here (if that is actually the reason why you are
using them).

Thanks,
Davidlohr

--
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/