diff --git a/core/lib/Drupal/Core/Routing/RouteBuilder.php b/core/lib/Drupal/Core/Routing/RouteBuilder.php
index e9692c5..e235e39 100644
--- a/core/lib/Drupal/Core/Routing/RouteBuilder.php
+++ b/core/lib/Drupal/Core/Routing/RouteBuilder.php
@@ -135,6 +135,11 @@ public function rebuild() {
       $this->lock->wait('router_rebuild');
 
       if ($this->routeBuilderIndicator->isRebuildStillNeeded()) {
+        // Per default the lock waits for 30 seconds. In case we haven't
+        // completed in the meantime, try to rebuild again. In case the
+        // rebuilding needs an incredible long time, and the callstack by this
+        // recursive call gets high, you already have a problem with your
+        // site being down for a long long time.
         $this->rebuild();
       }
 
@@ -220,6 +225,12 @@ public function getCollectionDuringRebuild() {
    */
   public function rebuildIfNeeded() {
     if ($this->routeBuilderIndicator->isRebuildNeeded()) {
+      // In case we need to rebuild the router, absolutely ensure that we need
+      // to rebuild the router. There is the small chance (which gets high in
+      // case there are a lot of requests to the page), that we have actually
+      // completed the rebuild in the meantime, but the internal indicator
+      // (for example state) is still static cached in this process. Therefore
+      // use an safe, but slow way, to check whether the rebuild is needed.
       if (!$this->routeBuilderIndicator->isRebuildStillNeeded()) {
         return FALSE;
       }