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

[Peter Zijlstra]

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.

> 
> >  	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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 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;
 }