linux-kernel - Re: [PATCH 4/8] v3 Allow memory_block to span multiple memory sections

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Date:	Tue, 20 Jul 2010 12:18:01 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Nathan Fontenot <nfont@...tin.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linuxppc-dev@...abs.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	greg@...ah.com
Subject: Re: [PATCH 4/8] v3 Allow memory_block to span multiple memory
 sections

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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