drm/i915: Repeat flush of idle work during suspend

The idle work handler is self-arming - if it detects that it needs to
run again it will queue itself from its work handler. Take greater care
when trying to drain the idle work, and double check that it is flushed.

The free worker has a similar issue where it is armed by an RCU task
which may be running concurrently with us.

This should hopefully help with the sporadic WARN_ON(dev_priv->gt.awake)
from i915_gem_suspend.

v2: Reuse drain_freed_objects.
v3: Don't try to flush the freed objects from the shrinker, as it may be
underneath the struct_mutex already.
v4: do while and comment upon the excess rcu_barrier in drain_freed_objects

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161223145804.6605-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -4264,8 +4264,14 @@ int i915_gem_suspend(struct drm_i915_private *dev_priv)
 
 	cancel_delayed_work_sync(&dev_priv->gpu_error.hangcheck_work);
 	cancel_delayed_work_sync(&dev_priv->gt.retire_work);
-	flush_delayed_work(&dev_priv->gt.idle_work);
-	flush_work(&dev_priv->mm.free_work);
+
+	/* As the idle_work is rearming if it detects a race, play safe and
+	 * repeat the flush until it is definitely idle.
+	 */
+	while (flush_delayed_work(&dev_priv->gt.idle_work))
+		;
+
+	i915_gem_drain_freed_objects(dev_priv);
 
 	/* Assert that we sucessfully flushed all the work and
 	 * reset the GPU back to its idle, low power state.