linux-kernel - Re: [cpuops cmpxchg double V3 4/5] Lockless (and preemptless) fastpaths for slub

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Date:	Fri, 25 Feb 2011 15:56:24 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Tejun Heo <tj@...nel.org>, akpm@...ux-foundation.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [cpuops cmpxchg double V3 4/5] Lockless (and preemptless)
	fastpaths for slub

* Christoph Lameter (cl@...ux.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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