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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, May 30, 2017 at 01:22:35PM +0200, Peter Zijlstra wrote:
> On Wed, May 24, 2017 at 11:30:18AM -0700, 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?
> > 
> > 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?
> 
> Mostly just lazy. This code path didn't present a problem with the lock
> ordering. Find the conversion below.

I know that feeling!  With this addition:

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> > >  	for_each_online_cpu(cpu) {
> > >  		struct perf_cpu_context *cpuctx;
> > >  		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
> > > @@ -8929,7 +8921,7 @@ perf_event_mux_interval_ms_store(struct
> > >  		cpu_function_call(cpu,
> > >  			(remote_function_f)perf_mux_hrtimer_restart, cpuctx);
> > >  	}
> > > -	put_online_cpus();
> > > +	cpus_read_unlock();
> > >  	mutex_unlock(&mux_interval_mutex);
> > > 
> > >  	return count;
> 
> 
> ---
> Subject: perf: Complete CPU hotplug conversion
> 
> Remove the last cpuc_read_lock() user in perf in favour of our internal
> state. This conversion is non critical as the lock ordering wasn't
> problematic but its nice to be consistent.
> 
> Reported-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/events/core.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 8d6acaeeea17..ad4f7f03b519 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -389,6 +389,16 @@ static atomic_t nr_switch_events __read_mostly;
>  static LIST_HEAD(pmus);
>  static DEFINE_MUTEX(pmus_lock);
>  static struct srcu_struct pmus_srcu;
> +
> +/*
> + * CPU hotplug handling, also see perf_event_{exit,init}_cpu().
> + *
> + * We use @pmus_lock to serialize PMU (un)registration against CPU hotplug,
> + * tracking the online state in @perf_online_mask and
> + * pmu->pmu_cpu_context->online. That latter is set while holding ctx->mutex
> + * and therefore holding ctx->mutex is sufficient to serialize against
> + * hotplug wrt cpuctx->online.
> + */
>  static cpumask_var_t perf_online_mask;
> 
>  /*
> @@ -8887,8 +8897,6 @@ perf_event_mux_interval_ms_show(struct device *dev,
>  	return snprintf(page, PAGE_SIZE-1, "%d\n", pmu->hrtimer_interval_ms);
>  }
> 
> -static DEFINE_MUTEX(mux_interval_mutex);
> -
>  static ssize_t
>  perf_event_mux_interval_ms_store(struct device *dev,
>  				 struct device_attribute *attr,
> @@ -8908,12 +8916,12 @@ perf_event_mux_interval_ms_store(struct device *dev,
>  	if (timer == pmu->hrtimer_interval_ms)
>  		return count;
> 
> -	mutex_lock(&mux_interval_mutex);
> +	/* use pmus_lock to order against hotplug and self serialize */
> +	mutex_lock(&pmus_lock);
>  	pmu->hrtimer_interval_ms = timer;
> 
>  	/* update all cpuctx for this PMU */
> -	cpus_read_lock();
> -	for_each_online_cpu(cpu) {
> +	for_each_cpu(cpu, perf_online_mask) {
>  		struct perf_cpu_context *cpuctx;
>  		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
>  		cpuctx->hrtimer_interval = ns_to_ktime(NSEC_PER_MSEC * timer);
> @@ -8921,8 +8929,7 @@ perf_event_mux_interval_ms_store(struct device *dev,
>  		cpu_function_call(cpu,
>  			(remote_function_f)perf_mux_hrtimer_restart, cpuctx);
>  	}
> -	cpus_read_unlock();
> -	mutex_unlock(&mux_interval_mutex);
> +	mutex_unlock(&pmus_lock);
> 
>  	return count;
>  }
>