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

[Michal Hocko <mhocko@kernel.org>]

On Thu 07-02-19 18:53:09, Tetsuo Handa wrote:
> 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>

This patch is ugly as hell! I do agree that for_each_cpu not working on
CONFIG_SMP=n sucks but why do we even care about lru_add_drain_all when
there is a single cpu? Why don't we simply do

diff --git a/mm/swap.c b/mm/swap.c
index aa483719922e..952f24b09070 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -660,6 +660,7 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
 
 static DEFINE_PER_CPU(struct work_struct, lru_add_drain_work);
 
+#ifdef CONFIG_SMP
 /*
  * Doesn't need any cpu hotplug locking because we do rely on per-cpu
  * kworkers being shut down before our page_alloc_cpu_dead callback is
@@ -702,6 +703,10 @@ void lru_add_drain_all(void)
 
 	mutex_unlock(&lock);
 }
+#else
+#define lru_add_drain_all() lru_add_drain()
+
+#endif
 
 /**
  * release_pages - batched put_page()
-- 
Michal Hocko
SUSE Labs