perf: Add the rcu_read_lock to protect the list_for_each_entry_rcu.

index 301079d..e2f2fa5 100644
@@ -2442,6 +2442,7 @@ static void perf_adjust_freq_unthr_context(struct perf_event_context *ctx,
 
 	raw_spin_lock(&ctx->lock);
 	perf_pmu_disable(ctx->pmu);
+	rcu_read_lock();
 
 	list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
 		if (event->state != PERF_EVENT_STATE_ACTIVE)

@@ -2483,6 +2484,7 @@ static void perf_adjust_freq_unthr_context(struct perf_event_context *ctx,
 		event->pmu->start(event, delta > 0 ? PERF_EF_RELOAD : 0);
 	}
 
+	rcu_read_unlock();
 	perf_pmu_enable(ctx->pmu);
 	raw_spin_unlock(&ctx->lock);
 }