drm/i915: Cancel retire_worker on parking

Replace the racy continuation check within retire_work with a definite
kill-switch on idling. The race was being exposed by gem_concurrent_blit
where the retire_worker would be terminated too early leaving us
spinning in debugfs/i915_drop_caches with nothing flushing the
retirement queue.

Although that the igt is trying to idle from one child while submitting
from another may be a contributing factor as to why  it runs so slowly...

v2: Use the non-sync version of cancel_delayed_work(), we only need to
stop it from being scheduled as we independently check whether now is
the right time to be parking.

Testcase: igt/gem_concurrent_blit
Fixes: 79ffac85 ("drm/i915: Invert the GEM wakeref hierarchy")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190507121108.18377-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_pm.c

@@ -30,15 +30,23 @@ static void idle_work_handler(struct work_struct *work)
 {
 	struct drm_i915_private *i915 =
 		container_of(work, typeof(*i915), gem.idle_work);
+	bool restart = true;
 
+	cancel_delayed_work(&i915->gem.retire_work);
 	mutex_lock(&i915->drm.struct_mutex);
 
 	intel_wakeref_lock(&i915->gt.wakeref);
-	if (!intel_wakeref_active(&i915->gt.wakeref) && !work_pending(work))
+	if (!intel_wakeref_active(&i915->gt.wakeref) && !work_pending(work)) {
 		i915_gem_park(i915);
+		restart = false;
+	}
 	intel_wakeref_unlock(&i915->gt.wakeref);
 
 	mutex_unlock(&i915->drm.struct_mutex);
+	if (restart)
+		queue_delayed_work(i915->wq,
+				   &i915->gem.retire_work,
+				   round_jiffies_up_relative(HZ));
 }
 
 static void retire_work_handler(struct work_struct *work)
@@ -52,7 +60,6 @@ static void retire_work_handler(struct work_struct *work)
 		mutex_unlock(&i915->drm.struct_mutex);
 	}
 
-	if (intel_wakeref_active(&i915->gt.wakeref))
 	queue_delayed_work(i915->wq,
 			   &i915->gem.retire_work,
 			   round_jiffies_up_relative(HZ));
@@ -140,7 +147,6 @@ void i915_gem_suspend(struct drm_i915_private *i915)
 	 * Assert that we successfully flushed all the work and
 	 * reset the GPU back to its idle, low power state.
 	 */
-	drain_delayed_work(&i915->gem.retire_work);
 	GEM_BUG_ON(i915->gt.awake);
 	flush_work(&i915->gem.idle_work);
 

drivers/gpu/drm/i915/selftests/mock_gem_device.c

@@ -58,7 +58,6 @@ static void mock_device_release(struct drm_device *dev)
 	i915_gem_contexts_lost(i915);
 	mutex_unlock(&i915->drm.struct_mutex);
 
-	drain_delayed_work(&i915->gem.retire_work);
 	flush_work(&i915->gem.idle_work);
 	i915_gem_drain_workqueue(i915);