Re: [RFC][PATCH 0/5] perf/tracing/cpuhotplug: Fix locking order

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 16, 2017 at 07:27:42AM -0700, Paul E. McKenney wrote:
> On Tue, May 16, 2017 at 05:46:06AM -0700, Paul E. McKenney wrote:

> > Something like this, yes.  Maybe even exactly like this.  ;-)
> 
> Ah, one thing I forgot...  If you are avoiding use of get_online_cpus(),
> you usually also have to be very careful about how you use things like
> cpu_online() and cpu_is_offline.

OK, so I think I got it wrong there. This hunk should close any race
between perf_pmu_register() and hotplug by tracking a global state
protected by pmus_lock. Thereby insuring the cpuctx->online state gets
initialized right.

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8993,6 +8993,8 @@ static int pmu_dev_alloc(struct pmu *pmu
 static struct lock_class_key cpuctx_mutex;
 static struct lock_class_key cpuctx_lock;
 
+static cpumask_var_t perf_online_mask;
+
 int perf_pmu_register(struct pmu *pmu, const char *name, int type)
 {
 	int cpu, ret;
@@ -9056,7 +9058,7 @@ int perf_pmu_register(struct pmu *pmu, c
 		lockdep_set_class(&cpuctx->ctx.mutex, &cpuctx_mutex);
 		lockdep_set_class(&cpuctx->ctx.lock, &cpuctx_lock);
 		cpuctx->ctx.pmu = pmu;
-		cpuctx->online = cpu_online(cpu);
+		cpuctx->online = cpumask_test_cpu(cpu, perf_online_mask);
 
 		__perf_mux_hrtimer_init(cpuctx, cpu);
 	}
@@ -10952,6 +10954,8 @@ static void __init perf_event_init_all_c
 	struct swevent_htable *swhash;
 	int cpu;
 
+	zalloc_cpumask_var(&perf_online_mask, GFP_KERNEL);
+
 	for_each_possible_cpu(cpu) {
 		swhash = &per_cpu(swevent_htable, cpu);
 		mutex_init(&swhash->hlist_mutex);
@@ -11011,6 +11015,7 @@ static void perf_event_exit_cpu_context(
 		cpuctx->online = 0;
 		mutex_unlock(&ctx->mutex);
 	}
+	cpumask_clear_cpu(cpu, perf_online_mask);
 	mutex_unlock(&pmus_lock);
 }
 #else
@@ -11028,6 +11033,7 @@ int perf_event_init_cpu(unsigned int cpu
 	perf_swevent_init_cpu(cpu);
 
 	mutex_lock(&pmus_lock);
+	cpumask_set_cpu(cpu, perf_online_mask);
 	list_for_each_entry(pmu, &pmus, entry) {
 		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
 		ctx = &cpuctx->ctx;

> > > ---
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -8997,7 +8997,6 @@ int perf_pmu_register(struct pmu *pmu, c
> > >  {
> > >  	int cpu, ret;
> > > 
> > > -	get_online_cpus();
> > >  	mutex_lock(&pmus_lock);
> > >  	ret = -ENOMEM;
> > >  	pmu->pmu_disable_count = alloc_percpu(int);
> > 
> > There is usually also some state check in here somewhere for the CPU
> > being offline from a perf perspective.  Such a check might already exist,
> > but I must plead ignorance of perf.

This just allocates per-cpu storage, that is per definition on the
possible mask and unrelated to the online mask.

> > > @@ -9093,7 +9092,6 @@ int perf_pmu_register(struct pmu *pmu, c
> > >  	ret = 0;
> > >  unlock:
> > >  	mutex_unlock(&pmus_lock);
> > > -	put_online_cpus();
> > > 
> > >  	return ret;
> > > 
> > > @@ -11002,10 +11000,9 @@ static void perf_event_exit_cpu_context(
> > >  	struct perf_cpu_context *cpuctx;
> > >  	struct perf_event_context *ctx;
> > >  	struct pmu *pmu;
> > > -	int idx;
> > > 
> > > -	idx = srcu_read_lock(&pmus_srcu);
> > > -	list_for_each_entry_rcu(pmu, &pmus, entry) {
> > > +	mutex_lock(&pmus_lock);
> > 
> > If the state change checked for by perf_pmu_register() needs to be also
> > guarded by ctx->mutex, this looks right to me.

Right, so we have two locks, pmus_lock that serializes hotplug vs
perf_pmu_register and ctx->lock that serializes find_get_context() vs
hotplug.

Together they ensure that if a PMU is observed, the PMU's cpuctx's have
the correct ->online state.

> > Just for completeness, the other style is to maintain separate per-CPU
> > state, in which case you would instead acquire pmus_lock, mark this
> > CPU off-limits to more perf_pmu_register() usage, release pmus_lock,
> > then clean up any old usage.

I'm not immediately seeing the other style, but per the above, I need
that too. Because the previous could race against hotplug if
perf_pmu_register() would observe cpu_online() as set after
perf_event_exit_cpu() or something.

With the above change any chance of a race is gone and we don't need to
worry about hotplug ordering too much.


Now I just need to do something about the swevent hash, because that's
got a hole in now.