Re: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpathsfor slub
From: Pekka Enberg
Date: Wed Nov 24 2010 - 02:16:23 EST
On Wed, Nov 24, 2010 at 1:51 AM, Christoph Lameter <cl@xxxxxxxxx> wrote:
> @@ -1737,23 +1770,53 @@ static __always_inline void *slab_alloc(
> {
> void **object;
> struct kmem_cache_cpu *c;
> - unsigned long flags;
> + unsigned long tid;
>
> if (slab_pre_alloc_hook(s, gfpflags))
> return NULL;
>
> - local_irq_save(flags);
> +redo:
> + /*
> + * Must read kmem_cache cpu data via this cpu ptr. Preemption is
> + * enabled. We may switch back and forth between cpus while
> + * reading from one cpu area. That does not matter as long
> + * as we end up on the original cpu again when doing the cmpxchg.
> + */
> c = __this_cpu_ptr(s->cpu_slab);
> +
> + /*
> + * The transaction ids are globally unique per cpu and per operation on
> + * a per cpu queue. Thus they can be guarantee that the cmpxchg_double
> + * occurs on the right processor and that there was no operation on the
> + * linked list in between.
> + */
> + tid = c->tid;
> + barrier();
You're using a compiler barrier after every load from c->tid. Why?
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