Re: [PATCH v2 08/10] perf/hisi_uncore: Avoid placing cpumask var on stack

[Mark Rutland]

On Wed, Apr 03, 2024 at 08:51:07PM +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/hisilicon/hisi_uncore_pmu.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/perf/hisilicon/hisi_uncore_pmu.c b/drivers/perf/hisilicon/hisi_uncore_pmu.c
> index 04031450d5fe..ccc9191ad1b6 100644
> --- a/drivers/perf/hisilicon/hisi_uncore_pmu.c
> +++ b/drivers/perf/hisilicon/hisi_uncore_pmu.c
> @@ -504,7 +504,6 @@ int hisi_uncore_pmu_offline_cpu(unsigned int cpu, struct hlist_node *node)
>  {
>  	struct hisi_pmu *hisi_pmu = hlist_entry_safe(node, struct hisi_pmu,
>  						     node);
> -	cpumask_t pmu_online_cpus;
>  	unsigned int target;
>  
>  	if (!cpumask_test_and_clear_cpu(cpu, &hisi_pmu->associated_cpus))
> @@ -518,9 +517,8 @@ int hisi_uncore_pmu_offline_cpu(unsigned int cpu, struct hlist_node *node)
>  	hisi_pmu->on_cpu = -1;
>  
>  	/* Choose a new CPU to migrate ownership of the PMU to */
> -	cpumask_and(&pmu_online_cpus, &hisi_pmu->associated_cpus,
> -		    cpu_online_mask);
> -	target = cpumask_any_but(&pmu_online_cpus, cpu);
> +	target = cpumask_any_and_but(&hisi_pmu->associated_cpus,
> +				     cpu_online_mask, cpu);
>  	if (target >= nr_cpu_ids)
>  		return 0;
>  
> -- 
> 2.27.0
>