On Mon, 23 Jul 2012 at 02:31 GMT, Gavin Shan <shangw@xxxxxxxxxxxxxxxxxx> wrote:
> sparse_index_init uses index_init_lock spinlock to protect root
> mem_section assignment. The lock is not necessary anymore because the
> function is called only during the boot (during paging init which
> is executed only from a single CPU) and from the hotplug code (by
> add_memory via arch_add_memory) which uses mem_hotplug_mutex.
>
> The lock has been introduced by 28ae55c9 (sparsemem extreme: hotplug
> preparation) and sparse_index_init was used only during boot at that
> time.
>
> Later when the hotplug code (and add_memory) was introduced there was
> no synchronization so it was possible to online more sections from
> the same root probably (though I am not 100% sure about that).
> The first synchronization has been added by 6ad696d2 (mm: allow memory
> hotplug and hibernation in the same kernel) which has been later
> replaced by the mem_hotplug_mutex - 20d6c96b (mem-hotplug: introduce
> {un}lock_memory_hotplug()).
>
> Let's remove the lock as it is not needed and it makes the code more
> confusing.
>
> [mhocko@xxxxxxx: changelog]
> Signed-off-by: Gavin Shan <shangw@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
Reviewed-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
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