[PATCH] mm/swap.c: workaround for_each_cpu() bug on UP kernel.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Since for_each_cpu(cpu, mask) added by commit 2d3854a37e8b767a ("cpumask:
introduce new API, without changing anything") did not evaluate the mask
argument if NR_CPUS == 1 due to CONFIG_SMP=n, lru_add_drain_all() is
hitting WARN_ON() at __flush_work() added by commit 4d43d395fed12463
("workqueue: Try to catch flush_work() without INIT_WORK().")
by unconditionally calling flush_work() [1].

We should fix for_each_cpu() etc. but we need enough grace period for
allowing people to test and fix unexpected behaviors including build
failures. Therefore, this patch temporarily duplicates flush_work() for
NR_CPUS == 1 case. This patch will be reverted after for_each_cpu() etc.
are fixed.

[1] https://lkml.kernel.org/r/18a30387-6aa5-6123-e67c-57579ecc3f38@roeck-us.net

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 mm/swap.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/swap.c b/mm/swap.c
index 4929bc1..e5e8e15 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -694,11 +694,16 @@ void lru_add_drain_all(void)
 			INIT_WORK(work, lru_add_drain_per_cpu);
 			queue_work_on(cpu, mm_percpu_wq, work);
 			cpumask_set_cpu(cpu, &has_work);
+#if NR_CPUS == 1
+			flush_work(work);
+#endif
 		}
 	}
 
+#if NR_CPUS != 1
 	for_each_cpu(cpu, &has_work)
 		flush_work(&per_cpu(lru_add_drain_work, cpu));
+#endif
 
 	mutex_unlock(&lock);
 }
-- 
1.8.3.1