drm/i915: Only sync tasklets once for recursive reset preparation

When setting up reset, we may need to recursively prepare an engine. In
which case we should only synchronously flush the tasklets on the outer
most call, the inner calls will then be inside an atomic section where
the tasklet will never be run (and so the sync will never complete).
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180516183355.10553-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -3036,7 +3036,7 @@ i915_gem_reset_prepare_engine(struct intel_engine_cs *engine)
 	 * Turning off the execlists->tasklet until the reset is over
 	 * prevents the race.
 	 */
-	tasklet_disable(&engine->execlists.tasklet);
+	__tasklet_disable_sync_once(&engine->execlists.tasklet);
 
 	/*
 	 * We're using worker to queue preemption requests from the tasklet in

drivers/gpu/drm/i915/i915_gem.h

@@ -26,6 +26,7 @@
 #define __I915_GEM_H__
 
 #include <linux/bug.h>
+#include <linux/interrupt.h>
 
 struct drm_i915_private;
 
@@ -72,4 +73,10 @@ struct drm_i915_private;
 void i915_gem_park(struct drm_i915_private *i915);
 void i915_gem_unpark(struct drm_i915_private *i915);
 
+static inline void __tasklet_disable_sync_once(struct tasklet_struct *t)
+{
+	if (atomic_inc_return(&t->count) == 1)
+		tasklet_unlock_wait(t);
+}
+
 #endif /* __I915_GEM_H__ */