Re: [PATCH] [RFC] watchdog/softlockup: Fix softlockup_stop_all() hungtask bug

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 17 Sep 2021 01:56:50 +0800 Jinhui Guo <guojinhui@huawei.com> wrote:

> If NR_CPUS equal to 1, it would trigger hungtask, it can be
> triggered by follow command:
> 	echo 0 > /proc/sys/kernel/watchdog
> 	echo 1 > /proc/sys/kernel/watchdog
> The hungtask stack:
> 	__schedule
> 	schedule
> 	schedule_timeout
> 	__wait_for_common
> 	softlockup_stop_fn
> 	lockup_detector_reconfigure
> 	proc_watchdog_common
> 	proc_watchdog
> 	proc_sys_call_handler
> 	vfs_write
> 	ksys_write
> The watchdog_allowed_mask is completely cleared when the
> watchdog is disabled. But the macro for_each_cpu() assume
> all masks are "1" when macro NR_CPUS equal to 1. It makes
> watchdog_allowed_mask not work at all.
> 
> ...
>
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -175,10 +175,11 @@ static inline int cpumask_any_distribute(const struct cpumask *srcp)
>  	return cpumask_first(srcp);
>  }
>  
> +/* It should check cpumask in some special case, such as watchdog */
>  #define for_each_cpu(cpu, mask)			\
> -	for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
> +	for ((cpu) = 0; (cpu) < 1 && test_bit(0, cpumask_bits(mask)); (cpu)++)
>  #define for_each_cpu_not(cpu, mask)		\
> -	for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
> +	for ((cpu) = 0; (cpu) < 1 && !test_bit(0, cpumask_bits(mask)); (cpu)++)
>  #define for_each_cpu_wrap(cpu, mask, start)	\
>  	for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)(start))
>  #define for_each_cpu_and(cpu, mask1, mask2)	\

x86_64 allnoconfig:

ld: arch/x86/kernel/cpu/cacheinfo.o: in function `populate_cache_leaves':
cacheinfo.c:(.text+0xa27): undefined reference to `cpu_llc_shared_map'
ld: cacheinfo.c:(.text+0xa49): undefined reference to `cpu_llc_shared_map'

Because the new for_each_cpu() now references `mask' and some code isn't
able to handle that change.  There are probably other instances of this
across all our architectures and configs.