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Date: Thu, 11 Oct 2007 09:46:25 -0700 (PDT)
From: Christoph Lameter <clameter@sgi.com>
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To: Akinobu Mita <akinobu.mita@gmail.com>
cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
       Pekka Enberg <penberg@cs.helsinki.fi>
Subject: Re: [PATCH -mm] slub: fix cpu hotplug offline/online path
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On Thu, 11 Oct 2007, Akinobu Mita wrote:

> > Why would get_cpu_slab not work?
> 
> 
>         case CPU_DEAD:
>         case CPU_DEAD_FROZEN:
>                 down_read(&slub_lock);
>                 list_for_each_entry(s, &slab_caches, list) {
>                         struct kmem_cache_cpu *c = get_cpu_slab(s, cpu);
> 
>                         local_irq_save(flags);
>                         __flush_cpu_slab(s, cpu);
>                         local_irq_restore(flags);
>                         free_kmem_cache_cpu(c, cpu);
>                         s->cpu_slab[cpu] = NULL; <----------------------
>                 }
>                 up_read(&slub_lock);
>                 break;
> 
> When CPU is offlined, cpu-hotplug notifier sets s->cpu_slab[cpu] = NULL.
> This means get_cpu_slab() always return NULL when CPU is being onlined.
> So I can't use get_cpu_slab to check whether kmem_cache_cpu_free
> initalization for the CPU has already been done or not.

If you have set it to NULL then the earlier kmem_cache_cpu structures has 
been freed. Why is it a problem to allocate another one when the cpu comes 
up again?