On Mon, 23 Jul 2012 at 02:31 GMT, Gavin Shan [off-list ref] 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@suse.cz: changelog]
Signed-off-by: Gavin Shan <redacted>
Reviewed-by: Michal Hocko <redacted>

Reviewed-by: Cong Wang <redacted>

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