Re: [PATCH 08/11] x86/tsc: Avoid synchronizing TSCs with multiple CPUs in parallel

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Dec 09, 2021 at 03:09:35PM +0000, David Woodhouse wrote:
> diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
> index 50a4515fe0ad..4ee247d89a49 100644
> --- a/arch/x86/kernel/tsc_sync.c
> +++ b/arch/x86/kernel/tsc_sync.c
> @@ -202,6 +202,7 @@ bool tsc_store_and_check_tsc_adjust(bool bootcpu)
>   * Entry/exit counters that make sure that both CPUs
>   * run the measurement code at once:
>   */
> +static atomic_t tsc_sync_cpu = ATOMIC_INIT(-1);
>  static atomic_t start_count;
>  static atomic_t stop_count;
>  static atomic_t skip_test;
> @@ -326,6 +327,8 @@ void check_tsc_sync_source(int cpu)
>  		atomic_set(&test_runs, 1);
>  	else
>  		atomic_set(&test_runs, 3);
> +
> +	atomic_set(&tsc_sync_cpu, cpu);
>  retry:
>  	/*
>  	 * Wait for the target to start or to skip the test:
> @@ -407,6 +410,10 @@ void check_tsc_sync_target(void)
>  	if (unsynchronized_tsc())
>  		return;
>  
> +	/* Wait for this CPU's turn */
> +	while (atomic_read(&tsc_sync_cpu) != cpu)
> +		cpu_relax();
> +
>  	/*
>  	 * Store, verify and sanitize the TSC adjust register. If
>  	 * successful skip the test.

This new atomic_t seems superfluous, there isn't any actual atomic
operation used.