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SubjectRe: [cpuops cmpxchg double V3 4/5] Lockless (and preemptless) fastpaths for slub
* Christoph Lameter (cl@linux.com) wrote:
> On Fri, 25 Feb 2011, Mathieu Desnoyers wrote:
>
> > > Index: linux-2.6/include/linux/slub_def.h
> > > ===================================================================
> > > --- linux-2.6.orig/include/linux/slub_def.h 2011-02-25 10:45:49.000000000 -0600
> > > +++ linux-2.6/include/linux/slub_def.h 2011-02-25 10:46:19.000000000 -0600
> > > @@ -35,7 +35,10 @@ enum stat_item {
> > > NR_SLUB_STAT_ITEMS };
> > >
> > > struct kmem_cache_cpu {
> > > - void **freelist; /* Pointer to first free per cpu object */
> > > + void **freelist; /* Pointer to next available object */
> > > +#ifdef CONFIG_CMPXCHG_LOCAL
> > > + unsigned long tid; /* Globally unique transaction id */
> > > +#endif
> >
> > There seem to be no strong guarantee that freelist is double-word aligned here.
>
> The struct kmem_cache_cpu allocation via alloc_percpu() specifies double
> word alignment. See the remainder of the code quoted by you:

So adding a comment on top of struct kmem_cache_cpu declaration might be
appropriate too, just in case it is ever defined elsewhere.

Thanks,

Mathieu

>
> > > +#ifdef CONFIG_CMPXCHG_LOCAL
> > > + /*
> > > + * Must align to double word boundary for the double cmpxchg instructions
> > > + * to work.
> > > + */
> > > + s->cpu_slab = __alloc_percpu(sizeof(struct kmem_cache_cpu), 2 * sizeof(void *));
> > > +#else
> > > + /* Regular alignment is sufficient */
> > > s->cpu_slab = alloc_percpu(struct kmem_cache_cpu);
> > > +#endif
> > > +
> > > + if (!s->cpu_slab)
> > > + return 0;
> > >
> > > - return s->cpu_slab != NULL;
> > > + init_kmem_cache_cpus(s);
> > > +
> > > + return 1;
> > > }

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com


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