linux-kernel - Re: [PREEMPT-RT] [PATCH] s390/cpum_sf: Remove superfluous SMP function call

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Message-ID: <20160405112336.GB6890@osiris>
Date:	Tue, 5 Apr 2016 13:23:36 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Sebastian Andrzej Siewior <sebastian.siewior@...utronix.de>
Cc:	Anna-Maria Gleixner <anna-maria@...utronix.de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
	rt@...utronix.de
Subject: Re: [PREEMPT-RT] [PATCH] s390/cpum_sf: Remove superfluous SMP
 function call

On Tue, Apr 05, 2016 at 01:13:06PM +0200, Sebastian Andrzej Siewior wrote:
> On 04/05/2016 12:49 PM, Heiko Carstens wrote:
> >> --- a/arch/s390/kernel/perf_cpum_sf.c
> >> +++ b/arch/s390/kernel/perf_cpum_sf.c
> >> @@ -1510,7 +1510,6 @@ static void cpumf_measurement_alert(stru
> >>  static int cpumf_pmu_notifier(struct notifier_block *self,
> >>  			      unsigned long action, void *hcpu)
> >>  {
> >> -	unsigned int cpu = (long) hcpu;
> >>  	int flags;
> >>
> >>  	/* Ignore the notification if no events are scheduled on the PMU.
> >> @@ -1523,11 +1522,15 @@ static int cpumf_pmu_notifier(struct not
> >>  	case CPU_ONLINE:
> >>  	case CPU_DOWN_FAILED:
> >>  		flags = PMC_INIT;
> >> -		smp_call_function_single(cpu, setup_pmc_cpu, &flags, 1);
> >> +		local_irq_disable();
> >> +		setup_pmc_cpu(&flags);
> >> +		local_irq_enable();
> >>  		break;
> > 
> > ...but at least the CPU_DOWN_FAILED callback will not necessarily be called
> > on the cpu that couldn't be brought offline.
> 
> I don't follow.

I was trying to say that if bringing a cpu down fails, then the cpu hotplug
notifier with CPU_DOWN_FAILED might be called on a cpu that is _not_ the
same cpu that was supposed to be brought offline.

Subsequently, in this case, the setup_pmc_cpu() call will be executed on
the wrong cpu.