[PATCH v3 2/2] x86: Fix /proc/cpuinfo cpumask warning

[Andrew Jones]

On Fri, Oct 14, 2022 at 05:58:45PM +0200, Andrew Jones wrote:
> Commit 78e5a3399421 ("cpumask: fix checking valid cpu range") has
> started issuing warnings[*] when cpu indices equal to nr_cpu_ids - 1
> are passed to cpumask_next* functions. seq_read_iter() and cpuinfo's
> start and next seq operations implement a pattern like
> 
>   n = cpumask_next(n - 1, mask);
>   show(n);
>   while (1) {
>       ++n;
>       n = cpumask_next(n - 1, mask);
>       if (n >= nr_cpu_ids)
>           break;
>       show(n);
>   }
> 
> which will issue the warning when reading /proc/cpuinfo. Ensure no
> warning is generated by validating the cpu index before calling
> cpumask_next().
> 
> [*] Warnings will only appear with DEBUG_PER_CPU_MAPS enabled.
> 
> Signed-off-by: Andrew Jones <ajones at ventanamicro.com>
> Cc: Yury Norov <yury.norov at gmail.com>
> ---
>  arch/x86/kernel/cpu/proc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
> index 099b6f0d96bd..de3f93ac6e49 100644
> --- a/arch/x86/kernel/cpu/proc.c
> +++ b/arch/x86/kernel/cpu/proc.c
> @@ -153,6 +153,9 @@ static int show_cpuinfo(struct seq_file *m, void *v)
>  
>  static void *c_start(struct seq_file *m, loff_t *pos)
>  {
> +	if (*pos == nr_cpu_ids)
> +		return NULL;
> +
>  	*pos = cpumask_next(*pos - 1, cpu_online_mask);
>  	if ((*pos) < nr_cpu_ids)
>  		return &cpu_data(*pos);
> -- 
> 2.37.3
>

Hi x86 maintainers,

I realize 78e5a3399421 has now been reverted, so this fix is no longer
urgent. I don't believe it's wrong, though, so if it's still of interest,
then please consider this a friendly ping.

Thanks,
drew