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

Mark Rutland
2024-04-03T14:16:51Z

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@arm.com>
> Signed-off-by: Dawei Li <dawei.li@shingroup.cn>

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@arm.com>

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