drm/i915/breadcrumbs: Disable interrupt bottom-half first on idling

Before walking the rbtree of waiters (marking them as complete and waking
them), decouple the interrupt handler. This prevents a race between the
missed waiter waking up and removing its intel_wait (which skips
checking the lock) and the interrupt handler dereferencing the
intel_wait. (Though we do not expect to encounter waiters during idle!)

Fixes: e1c0c91b ("drm/i915: Wake up all waiters before idling")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170315210726.12095-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_breadcrumbs.c

@@ -179,7 +179,7 @@ void __intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine)
 void intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine)
 {
 	struct intel_breadcrumbs *b = &engine->breadcrumbs;
-	struct intel_wait *wait, *n;
+	struct intel_wait *wait, *n, *first;
 
 	if (!b->irq_armed)
 		return;
@@ -190,18 +190,19 @@ void intel_engine_disarm_breadcrumbs(struct intel_engine_cs *engine)
 	 */
 
 	spin_lock_irq(&b->rb_lock);
+
+	spin_lock(&b->irq_lock);
+	first = fetch_and_zero(&b->irq_wait);
+	__intel_engine_disarm_breadcrumbs(engine);
+	spin_unlock(&b->irq_lock);
+
 	rbtree_postorder_for_each_entry_safe(wait, n, &b->waiters, node) {
 		RB_CLEAR_NODE(&wait->node);
-		if (wake_up_process(wait->tsk) && wait == b->irq_wait)
+		if (wake_up_process(wait->tsk) && wait == first)
 			missed_breadcrumb(engine);
 	}
 	b->waiters = RB_ROOT;
 
-	spin_lock(&b->irq_lock);
-	b->irq_wait = NULL;
-	__intel_engine_disarm_breadcrumbs(engine);
-	spin_unlock(&b->irq_lock);
-
 	spin_unlock_irq(&b->rb_lock);
 }