Re: [PATCH v2 05/10] perf/arm_dsu: Avoid placing cpumask var on stack

[Mark Rutland]

On Wed, Apr 03, 2024 at 08:51:04PM +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/arm_dsu_pmu.c | 19 ++++++-------------
>  1 file changed, 6 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/perf/arm_dsu_pmu.c b/drivers/perf/arm_dsu_pmu.c
> index bae3ca37f846..adc0bbb5fafe 100644
> --- a/drivers/perf/arm_dsu_pmu.c
> +++ b/drivers/perf/arm_dsu_pmu.c
> @@ -230,15 +230,6 @@ static const struct attribute_group *dsu_pmu_attr_groups[] = {
>  	NULL,
>  };
>  
> -static int dsu_pmu_get_online_cpu_any_but(struct dsu_pmu *dsu_pmu, int cpu)
> -{
> -	struct cpumask online_supported;
> -
> -	cpumask_and(&online_supported,
> -			 &dsu_pmu->associated_cpus, cpu_online_mask);
> -	return cpumask_any_but(&online_supported, cpu);
> -}
> -
>  static inline bool dsu_pmu_counter_valid(struct dsu_pmu *dsu_pmu, u32 idx)
>  {
>  	return (idx < dsu_pmu->num_counters) ||
> @@ -827,14 +818,16 @@ static int dsu_pmu_cpu_online(unsigned int cpu, struct hlist_node *node)
>  
>  static int dsu_pmu_cpu_teardown(unsigned int cpu, struct hlist_node *node)
>  {
> -	int dst;
> -	struct dsu_pmu *dsu_pmu = hlist_entry_safe(node, struct dsu_pmu,
> -						   cpuhp_node);
> +	struct dsu_pmu *dsu_pmu;
> +	unsigned int dst;
> +
> +	dsu_pmu = hlist_entry_safe(node, struct dsu_pmu, cpuhp_node);
>  
>  	if (!cpumask_test_and_clear_cpu(cpu, &dsu_pmu->active_cpu))
>  		return 0;
>  
> -	dst = dsu_pmu_get_online_cpu_any_but(dsu_pmu, cpu);
> +	dst = cpumask_any_and_but(&dsu_pmu->associated_cpus,
> +				  cpu_online_mask, cpu);
>  	/* If there are no active CPUs in the DSU, leave IRQ disabled */
>  	if (dst >= nr_cpu_ids)
>  		return 0;
> -- 
> 2.27.0
>