linux-kernel - Re: [patch V3 23/32] perf/tracing/cpuhotplug: Fix locking order

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Date:   Wed, 24 May 2017 20:47:00 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [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