Re: [PATCH v5 17/51] x86/tsc: Fold native_calibrate_cpu() into recalibrate_cpu_khz()

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The refactoring in `recalibrate_cpu_khz()` introduces a race condition where the global variable `cpu_khz` is temporarily set to 0 for up to 150ms during runtime CPU frequency recalibration. This exposes the system to invalid state reads.
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commit 34cf1b98d726e6c329bdefbd5c9712901d8b4bc8
Author: Sean Christopherson <[email protected]>

x86/tsc: Fold native_calibrate_cpu() into recalibrate_cpu_khz()

This commit folds the implementation of native_calibrate_cpu() directly into
recalibrate_cpu_khz() to remove an extra CONFIG_SMP=n ifdef block. This change
aims to clarify the intentional use of early versus late calibration routines.

> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 9764ac758081f..6ed6f8f012ebf 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
[ ... ]
> @@ -944,7 +929,9 @@ void recalibrate_cpu_khz(void)
>  	if (WARN_ON_ONCE(cpu_feature_enabled(X86_FEATURE_TSC_KNOWN_FREQ)))
>  		return;
>  
> -	cpu_khz = native_calibrate_cpu();
> +	cpu_khz = native_calibrate_cpu_early();
> +	if (!cpu_khz)
> +		cpu_khz = native_calibrate_cpu_late();

[Severity: Medium]
Does this expose a race condition by temporarily assigning 0 to the global
cpu_khz variable?

On a UP kernel during cpufreq transitions, if early calibration fails, the
global cpu_khz is directly assigned 0. It remains 0 while
native_calibrate_cpu_late() executes, which can take ~150ms and enables
interrupts.

Could this lead to sysfs readers via arch_freq_get_on_cpu() observing a 0
frequency, or the NMI watchdog being misconfigured with a 0 sample period if
toggled during this window? 

The previous implementation used a local variable inside native_calibrate_cpu()
to hold the intermediate state, preventing concurrent readers from seeing an
invalid 0 value before the slow fallback calibration completes.

>  	tsc_khz = native_calibrate_tsc();
>  	if (tsc_khz == 0)
>  		tsc_khz = cpu_khz;

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Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=17