linux-kernel - Re: [PATCH v2 02/18] KVM: x86: Use __free(put_cpufreq

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Message-ID: <aK8Sd30K64mbN1Nt@google.com>
Date: Wed, 27 Aug 2025 07:13:11 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Zihuan Zhang <zhangzihuan@...inos.cn>
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Subject: Re: [PATCH v2 02/18] KVM: x86: Use __free(put_cpufreq_policy) for
 policy reference

On Wed, Aug 27, 2025, Zihuan Zhang wrote:
> Replace the manual cpufreq_cpu_put() with __free(put_cpufreq_policy)
> annotation for policy references. This reduces the risk of reference
> counting mistakes and aligns the code with the latest kernel style.
> 
> No functional change intended.
> 
> Signed-off-by: Zihuan Zhang <zhangzihuan@...inos.cn>
> ---
>  arch/x86/kvm/x86.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a1c49bc681c4..2a825f4ec701 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9492,16 +9492,14 @@ static void kvm_timer_init(void)
>  		max_tsc_khz = tsc_khz;
>  
>  		if (IS_ENABLED(CONFIG_CPU_FREQ)) {
> -			struct cpufreq_policy *policy;
> +			struct cpufreq_policy *policy __free(put_cpufreq_policy);
>  			int cpu;
>  
>  			cpu = get_cpu();
>  			policy = cpufreq_cpu_get(cpu);
> -			if (policy) {
> -				if (policy->cpuinfo.max_freq)
> -					max_tsc_khz = policy->cpuinfo.max_freq;
> -				cpufreq_cpu_put(policy);
> -			}
> +			if (policy && policy->cpuinfo.max_freq)
> +				max_tsc_khz = policy->cpuinfo.max_freq;
> +
>  			put_cpu();

Hmm, this is technically buggy.  __free() won't invoke put_cpufreq_policy() until
policy goes out of scope, and so using __free() means the code is effectively:

		if (IS_ENABLED(CONFIG_CPU_FREQ)) {
			struct cpufreq_policy *policy;
			int cpu;

			cpu = get_cpu();
			policy = cpufreq_cpu_get(cpu);
			if (policy && policy->cpuinfo.max_freq)
				max_tsc_khz = policy->cpuinfo.max_freq;
			put_cpu();

			if (policy)
				cpufreq_cpu_put(policy);
		}

That's "fine" because the policy isn't truly referenced after preemption is
disabled, the lifecycle of the policy doesn't rely on preemption being disabled,
and KVM doesn't actually care which CPU is used to get the max frequency, i.e.
this would technically be "fine" too:

		if (IS_ENABLED(CONFIG_CPU_FREQ)) {
			struct cpufreq_policy *policy;
			int cpu;

			cpu = get_cpu();
			policy = cpufreq_cpu_get(cpu);
			put_cpu();

			if (policy && policy->cpuinfo.max_freq)
				max_tsc_khz = policy->cpuinfo.max_freq;

			if (policy)
				cpufreq_cpu_put(policy);
		}

But given that the code we have today is perfectly readable, I don't see any
reason to switch to __free() given that's it's technically flawed.  So I'm very
strongly inclined to skip this patch and keep things as-is.