Re: [tip:perf/urgent] perf/x86/mbm: Implement RMID recycling

[Matt Fleming <matt@codeblueprint.co.uk>]

On Mon, 21 Mar, at 02:53:04AM, tip-bot for Vikas Shivappa wrote:
> @@ -489,6 +496,22 @@ static u32 intel_cqm_xchg_rmid(struct perf_event *group, u32 rmid)
>  
>  	raw_spin_unlock_irq(&cache_lock);
>  
> +	/*
> +	 * If the allocation is for mbm, init the mbm stats.
> +	 * Need to check if each event in the group is mbm event
> +	 * because there could be multiple type of events in the same group.
> +	 */
> +	if (__rmid_valid(rmid)) {
> +		event = group;
> +		if (is_mbm_event(event->attr.config))
> +			init_mbm_sample(rmid, event->attr.config);
> +
> +		list_for_each_entry(event, head, hw.cqm_group_entry) {
> +			if (is_mbm_event(event->attr.config))
> +				init_mbm_sample(rmid, event->attr.config);
> +		}
> +	}
> +
>  	return old_rmid;
>  }
>  

You're calling init_mbm_sample() without holding cache_lock. Won't
this potentially trash the existing value in MSR_IA32_QM_EVTSEL, if
say, we're reading the counter at the same time as the recycling
worker is running?