Hi,
在 2025/8/27 22:13, Sean Christopherson 写道:
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 <zhangzih...@kylinos.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.
Yes, this will indeed change the execution order.
Can you accept that? Personally, I don’t think it’s ideal either.
if (IS_ENABLED(CONFIG_CPU_FREQ)) {
int cpu;
cpu = get_cpu();
{
struct cpufreq_policy *policy
__free(put_cpufreq_policy) = cpufreq_cpu_get(cpu);
if (policy && policy->cpuinfo.max_freq)
max_tsc_khz = policy->cpuinfo.max_freq;
}
put_cpu();
}
Other places may also have the same issue,
maybe we should consider introducing a macro to handle this properly,
so that initialization and cleanup are well defined without changing
the existing order unexpected.
like this:
#define WITH_CPUFREQ_POLICY(cpu) {\
for(struct cpufreq_policy *policy __free(put_cpufreq_policy) = \
cpufreq_cpu_get(cpu); \
policy;)
Then Use it:
if (IS_ENABLED(CONFIG_CPU_FREQ)) {
int cpu;
cpu = get_cpu();
WITH_CPUFREQ_POLICY(cpu){
if (policy->cpuinfo.max_freq)
max_tsc_khz = policy->cpuinfo.max_freq;
}
put_cpu();
}
