drm/i915/selftests: Synchronize checking active status with retirement

If retirement is running on another thread, we may inspect the status of
the i915_active before its retirement callback is complete. As we expect
it to be running synchronously, we can wait for any callback to complete
by acquiring the i915_active.mutex.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022112111.9317-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/selftest_engine_heartbeat.c

@@ -49,12 +49,20 @@ static struct pulse *pulse_create(void)
 	return p;
 }
 
+static void pulse_unlock_wait(struct pulse *p)
+{
+	mutex_lock(&p->active.mutex);
+	mutex_unlock(&p->active.mutex);
+}
+
 static int __live_idle_pulse(struct intel_engine_cs *engine,
 			     int (*fn)(struct intel_engine_cs *cs))
 {
 	struct pulse *p;
 	int err;
 
+	GEM_BUG_ON(!intel_engine_pm_is_awake(engine));
+
 	p = pulse_create();
 	if (!p)
 		return -ENOMEM;
@@ -73,16 +81,21 @@ static int __live_idle_pulse(struct intel_engine_cs *engine,
 	i915_active_release(&p->active);
 
 	GEM_BUG_ON(i915_active_is_idle(&p->active));
+	GEM_BUG_ON(llist_empty(&engine->barrier_tasks));
 
 	err = fn(engine);
 	if (err)
 		goto out;
 
+	GEM_BUG_ON(!llist_empty(&engine->barrier_tasks));
+
 	if (intel_gt_retire_requests_timeout(engine->gt, HZ / 5)) {
 		err = -ETIME;
 		goto out;
 	}
 
+	pulse_unlock_wait(p); /* synchronize with the retirement callback */
+
 	if (!i915_active_is_idle(&p->active)) {
 		pr_err("%s: heartbeat pulse did not flush idle tasks\n",
 		       engine->name);