mm/sparse: remove index_init_lock

sparse_index_init() uses the index_init_lock spinlock to protect root
mem_section assignment.  The lock is not necessary anymore because the
function is called only during 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 was 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 was 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 <shangw@linux.vnet.ibm.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/sparse.c

@@ -77,7 +77,6 @@ static struct mem_section noinline __init_refok *sparse_index_alloc(int nid)
 
 static int __meminit sparse_index_init(unsigned long section_nr, int nid)
 {
-	static DEFINE_SPINLOCK(index_init_lock);
 	unsigned long root = SECTION_NR_TO_ROOT(section_nr);
 	struct mem_section *section;
 	int ret = 0;
@@ -88,20 +87,9 @@ static int __meminit sparse_index_init(unsigned long section_nr, int nid)
 	section = sparse_index_alloc(nid);
 	if (!section)
 		return -ENOMEM;
-	/*
-	 * This lock keeps two different sections from
-	 * reallocating for the same index
-	 */
-	spin_lock(&index_init_lock);
-
-	if (mem_section[root]) {
-		ret = -EEXIST;
-		goto out;
-	}
 
 	mem_section[root] = section;
-out:
-	spin_unlock(&index_init_lock);
+
 	return ret;
 }
 #else /* !SPARSEMEM_EXTREME */