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.
