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

From: William Kucharski <william.kucharski@oracle.com>
To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Chris Metcalf <chris.d.metcalf@gmail.com>,
	Rusty Russell <rusty@rustcorp.com.au>,
	linux-mm@kvack.org, Guenter Roeck <linux@roeck-us.net>
Subject: Re: [PATCH] mm/swap.c: workaround for_each_cpu() bug on UP kernel.
Date: Thu, 7 Feb 2019 07:18:15 -0700	[thread overview]
Message-ID: <1F007F77-805D-43D6-81F6-816E836BF11E@oracle.com> (raw)
In-Reply-To: <1549533189-9177-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>

Would you mind adding a comment explaining this?

That way if, for some reason, the patch isn't reverted in a timely manner, anyone
stumbling upon the code knows why it's done the way it is without having to track
down this mail thread.

Reviewed by: William Kucharski <william.kucharski@oracle.com> 

> On Feb 7, 2019, at 2:53 AM, Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> 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>
> ---
> 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
> 