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    SubjectRe: [cpuops cmpxchg double V3 4/5] Lockless (and preemptless) fastpaths for slub
    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:

    > > +#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;
    > > }


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