Re: [PATCH v2 02/10] perf/alibaba_uncore_drw: Avoid placing cpumask var on stack

[Mark Rutland]

On Wed, Apr 03, 2024 at 08:51:01PM +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>

I don't think we need to explain all the pitfalls of the approach we haven't
taken. Could we please simplify this down to:

Could we please get rid of the bit that says we should "always use the
*cpumask_var API(s)", and simplify the commit message down to:

| perf/alibaba_uncore_drw: Avoid placing cpumask on the stack
| 
| In general it's preferable to avoid placing cpumasks on the stack, as
| for large values of NR_CPUS these can consume significant amounts of
| stack space and make stack overflows more likely.
| 
| Use cpumask_any_and_but() to avoid the need for a temporary cpumask on
| the stack.

The logic looks good to me, so with that commit message:

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

Mark.

> ---
>  drivers/perf/alibaba_uncore_drw_pmu.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/perf/alibaba_uncore_drw_pmu.c b/drivers/perf/alibaba_uncore_drw_pmu.c
> index a9277dcf90ce..d4d14b65c4a5 100644
> --- a/drivers/perf/alibaba_uncore_drw_pmu.c
> +++ b/drivers/perf/alibaba_uncore_drw_pmu.c
> @@ -746,18 +746,14 @@ static int ali_drw_pmu_offline_cpu(unsigned int cpu, struct hlist_node *node)
>  	struct ali_drw_pmu_irq *irq;
>  	struct ali_drw_pmu *drw_pmu;
>  	unsigned int target;
> -	int ret;
> -	cpumask_t node_online_cpus;
>  
>  	irq = hlist_entry_safe(node, struct ali_drw_pmu_irq, node);
>  	if (cpu != irq->cpu)
>  		return 0;
>  
> -	ret = cpumask_and(&node_online_cpus,
> -			  cpumask_of_node(cpu_to_node(cpu)), cpu_online_mask);
> -	if (ret)
> -		target = cpumask_any_but(&node_online_cpus, cpu);
> -	else
> +	target = cpumask_any_and_but(cpumask_of_node(cpu_to_node(cpu)),
> +				     cpu_online_mask, cpu);
> +	if (target >= nr_cpu_ids)
>  		target = cpumask_any_but(cpu_online_mask, cpu);
>  
>  	if (target >= nr_cpu_ids)
> -- 
> 2.27.0
>