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

Mark Rutland
2024-04-03T14:38:35Z

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

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@arm.com>

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