Re: [PATCH V4 2/2] perf/x86: add sysfs entry to freeze counter on SMI

[Andi Kleen <ak@linux.intel.com>]

> +	if (x86_pmu.attr_freeze_on_smi == val)
> +		return count;
> +
> +	mutex_lock(&freeze_on_smi_mutex);

I don't think the mutex is really needed, but if it was it would 
need to include the previous check too to be atomic.

> +
> +	get_online_cpus();
> +
> +	flip_smm_bit(&val);
> +	smp_call_function(flip_smm_bit, &val, 1);
> +
> +	put_online_cpus();

This is on_each_cpu() 

> +
> +	x86_pmu.attr_freeze_on_smi = val;
> +
> +	mutex_unlock(&freeze_on_smi_mutex);


-Andi