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

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Date:   Wed, 24 May 2017 14:10:48 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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, May 24, 2017 at 08:47:00PM +0200, Thomas Gleixner wrote:
> 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 :)

Another reason might be a desire to avoid contention on pmus_lock,
if this function is called often.  If that is the case, I cannot
resist suggesting percpu_rw_sem.  ;-)

							Thanx, Paul