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

[Guenter Roeck <linux@roeck-us.net>]

On 2/7/19 1:53 AM, 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>

I agree with the fix/workaround. I tried a complete build with fixed macros,
but that doesn't work because (at least) x86 assumes that the "mask" parameter
is _not_ evaluated for non-SMP builds - arch/x86/kernel/cpu/cacheinfo.c
passes cpu_llc_shared_mask(cpu) as parameter, and that is only defined
for SMP builds.

On the plus side, I did not find any other issues, but that doesn't mean
much since various build and boot tests in -next fail for other reasons.

Acked-by: Guenter Roeck <linux@roeck-us.net>

Guenter

> ---
>   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);
>   }
>