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Subject: Re: [PATCH 4/8] v3 Allow memory_block to span multiple memory
 sections
From: Dave Hansen <dave@linux.vnet.ibm.com>
In-Reply-To: <4C451E1C.8070907@austin.ibm.com>
References: <4C451BF5.50304@austin.ibm.com>
	 <4C451E1C.8070907@austin.ibm.com>
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Date: Tue, 20 Jul 2010 12:18:01 -0700
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Sender: owner-linux-mm@kvack.org
To: Nathan Fontenot <nfont@austin.ibm.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@ozlabs.org, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, greg@kroah.com
List-ID: <linux-mm.kvack.org>

On Mon, 2010-07-19 at 22:55 -0500, Nathan Fontenot wrote:
> +static int add_memory_section(int nid, struct mem_section *section,
> +                       unsigned long state, enum mem_add_context context)
> +{
> +       struct memory_block *mem;
> +       int ret = 0;
> +
> +       mem = find_memory_block(section);
> +       if (mem) {
> +               atomic_inc(&mem->section_count);
> +               kobject_put(&mem->sysdev.kobj);
> +       } else
> +               ret = init_memory_block(&mem, section, state);
> +
>         if (!ret) {
> -               if (context == HOTPLUG)
> +               if (context == HOTPLUG &&
> +                   atomic_read(&mem->section_count) == sections_per_block)
>                         ret = register_mem_sect_under_node(mem, nid);
>         } 

I think the atomic_inc() can race with the atomic_dec_and_test() in
remove_memory_block().

Thread 1 does:

	mem = find_memory_block(section);

Thread 2 does 

	atomic_dec_and_test(&mem->section_count);

and destroys the memory block,  Thread 1 runs again:
	
       if (mem) {
               atomic_inc(&mem->section_count);
               kobject_put(&mem->sysdev.kobj);
       } else

but now mem got destroyed by Thread 2.  You probably need to change
find_memory_block() to itself take a reference, and to use
atomic_inc_unless().

-- Dave

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