mm: hugetlb: fix non-atomic enqueue of huge page

commit b0365c8d upstream.

If a huge page is enqueued under the protection of hugetlb_lock, then the
operation is atomic and safe.
Signed-off-by: Hillf Danton <dhillf@gmail.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

mm/hugetlb.c

@@ -901,7 +901,6 @@ static int gather_surplus_pages(struct hstate *h, int delta)
 	h->resv_huge_pages += delta;
 	ret = 0;
 
-	spin_unlock(&hugetlb_lock);
 	/* Free the needed pages to the hugetlb pool */
 	list_for_each_entry_safe(page, tmp, &surplus_list, lru) {
 		if ((--needed) < 0)
@@ -915,6 +914,7 @@ static int gather_surplus_pages(struct hstate *h, int delta)
 		VM_BUG_ON(page_count(page));
 		enqueue_huge_page(h, page);
 	}
+	spin_unlock(&hugetlb_lock);
 
 	/* Free unnecessary surplus pages to the buddy allocator */
 free: