mm/hugetlb.c: add cond_resched_lock() in return_unused_surplus_pages()

soft lockup in freeing gigantic hugepage fixed in commit 55f67141a8 "mm:
hugetlb: fix softlockup when a large number of hugepages are freed." can
happen in return_unused_surplus_pages(), so let's fix it.

Signed-off-by: Masayoshi Mizuma <[email protected]>
Signed-off-by: Naoya Horiguchi <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Aneesh Kumar <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/hugetlb.c

@@ -1172,6 +1172,7 @@ static void return_unused_surplus_pages(struct hstate *h,
 	while (nr_pages--) {
 		if (!free_pool_huge_page(h, &node_states[N_MEMORY], 1))
 			break;
+		cond_resched_lock(&hugetlb_lock);
 	}
 }