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SubjectRe: [patch V3 23/32] perf/tracing/cpuhotplug: Fix locking order
On Wed, 24 May 2017, Paul E. McKenney wrote:
> > @@ -8920,7 +8912,7 @@ perf_event_mux_interval_ms_store(struct
> > pmu->hrtimer_interval_ms = timer;
> >
> > /* update all cpuctx for this PMU */
> > - get_online_cpus();
> > + cpus_read_lock();
>
> OK, I'll bite...
>
> Why is this piece using cpus_read_lock() instead of pmus_lock?
>
> My guess is for the benefit of the cpu_function_call() below, but if
> the code instead cycled through the perf_online_mask, wouldn't any
> CPU selected be guaranteed to be online?

Indeed.

> Or is there some reason that it would be necessary to specially handle
> CPUs that perf does not consider to be active, but that are still at
> least partway online?

I have to delegate that question to Peter :)

Thanks,

tglx

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