Re: [PATCH v2 10/10] perf/thunderx2: Avoid placing cpumask var on stack

[Mark Rutland]

On Wed, Apr 03, 2024 at 08:51:09PM +0800, Dawei Li wrote:
> For CONFIG_CPUMASK_OFFSTACK=y kernel, explicit allocation of cpumask
> variable on stack is not recommended since it can cause potential stack
> overflow.
> 
> Instead, kernel code should always use *cpumask_var API(s) to allocate
> cpumask var in config-neutral way, leaving allocation strategy to
> CONFIG_CPUMASK_OFFSTACK.
> 
> But dynamic allocation in cpuhp's teardown callback is somewhat problematic
> for if allocation fails(which is unlikely but still possible):
> - If -ENOMEM is returned to caller, kernel crashes for non-bringup
>   teardown;
> - If callback pretends nothing happened and returns 0 to caller, it may
>   trap system into an in-consisitent/compromised state;
> 
> Use newly-introduced cpumask_any_and_but() to address all issues above.
> It eliminates usage of temporary cpumask var in generic way, no matter how
> the cpumask var is allocated.
> 
> Suggested-by: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: Dawei Li <dawei.li@xxxxxxxxxxxx>

The logic looks good to me, but I'd like the commit message updated the same as
per my comment on patch 2.

With that commit message:

Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>

Mark.

> ---
>  drivers/perf/thunderx2_pmu.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/perf/thunderx2_pmu.c b/drivers/perf/thunderx2_pmu.c
> index e16d10c763de..b3377b662ffc 100644
> --- a/drivers/perf/thunderx2_pmu.c
> +++ b/drivers/perf/thunderx2_pmu.c
> @@ -932,9 +932,8 @@ static int tx2_uncore_pmu_online_cpu(unsigned int cpu,
>  static int tx2_uncore_pmu_offline_cpu(unsigned int cpu,
>  		struct hlist_node *hpnode)
>  {
> -	int new_cpu;
>  	struct tx2_uncore_pmu *tx2_pmu;
> -	struct cpumask cpu_online_mask_temp;
> +	unsigned int new_cpu;
>  
>  	tx2_pmu = hlist_entry_safe(hpnode,
>  			struct tx2_uncore_pmu, hpnode);
> @@ -945,11 +944,8 @@ static int tx2_uncore_pmu_offline_cpu(unsigned int cpu,
>  	if (tx2_pmu->hrtimer_callback)
>  		hrtimer_cancel(&tx2_pmu->hrtimer);
>  
> -	cpumask_copy(&cpu_online_mask_temp, cpu_online_mask);
> -	cpumask_clear_cpu(cpu, &cpu_online_mask_temp);
> -	new_cpu = cpumask_any_and(
> -			cpumask_of_node(tx2_pmu->node),
> -			&cpu_online_mask_temp);
> +	new_cpu = cpumask_any_and_but(cpumask_of_node(tx2_pmu->node),
> +				      cpu_online_mask, cpu);
>  
>  	tx2_pmu->cpu = new_cpu;
>  	if (new_cpu >= nr_cpu_ids)
> -- 
> 2.27.0
>