diff --git a/core/lib/Drupal/Core/Routing/RouteBuilder.php b/core/lib/Drupal/Core/Routing/RouteBuilder.php
index a4b17e2..e235e39 100644
--- a/core/lib/Drupal/Core/Routing/RouteBuilder.php
+++ b/core/lib/Drupal/Core/Routing/RouteBuilder.php
@@ -133,6 +133,16 @@ public function rebuild() {
       // We choose to block here since otherwise the routes might not be
       // available, resulting in a 404.
       $this->lock->wait('router_rebuild');
+
+      if ($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();
+      }
+
       return FALSE;
     }
 
@@ -215,6 +225,15 @@ 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;
+      }
       return $this->rebuild();
     }
     return FALSE;
diff --git a/core/lib/Drupal/Core/Routing/RouteBuilderIndicator.php b/core/lib/Drupal/Core/Routing/RouteBuilderIndicator.php
index 5f06120..da99bf2 100644
--- a/core/lib/Drupal/Core/Routing/RouteBuilderIndicator.php
+++ b/core/lib/Drupal/Core/Routing/RouteBuilderIndicator.php
@@ -52,4 +52,14 @@ public function setRebuildDone() {
     $this->state->set(static::REBUILD_NEEDED, FALSE);
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function isRebuildStillNeeded() {
+    // To absolutely ensure that the router rebuild is required, reset the cache
+    // in case they were set by another process.
+    $this->state->resetCache();
+    return $this->isRebuildNeeded();
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Routing/RouteBuilderIndicatorInterface.php b/core/lib/Drupal/Core/Routing/RouteBuilderIndicatorInterface.php
index 68ce5b8..fcd8495 100644
--- a/core/lib/Drupal/Core/Routing/RouteBuilderIndicatorInterface.php
+++ b/core/lib/Drupal/Core/Routing/RouteBuilderIndicatorInterface.php
@@ -36,4 +36,16 @@ public function setRebuildDone();
    */
   public function isRebuildNeeded();
 
+  /**
+   * Checks if the router rebuild really needed.
+   *
+   * This method should not rely on any static caching.
+   *
+   * This method is called during route rebuilding if acquiring a lock failed.
+   *
+   * @return bool
+   *   TRUE if the router still needs to be rebuilt, FALSE if not.
+   */
+  public function isRebuildStillNeeded();
+
 }
diff --git a/core/tests/Drupal/Tests/Core/Routing/RouteBuilderTest.php b/core/tests/Drupal/Tests/Core/Routing/RouteBuilderTest.php
index b0efe77..d25cc58 100644
--- a/core/tests/Drupal/Tests/Core/Routing/RouteBuilderTest.php
+++ b/core/tests/Drupal/Tests/Core/Routing/RouteBuilderTest.php
@@ -278,9 +278,13 @@ public function testRebuildIfNecessary() {
     $this->routeBuilderIndicator->expects($this->once())
                 ->method('setRebuildDone');
 
-    $this->routeBuilderIndicator->expects($this->exactly(2))
+    $this->routeBuilderIndicator->expects($this->exactly(3))
                 ->method('isRebuildNeeded')
-                ->will($this->onConsecutiveCalls(TRUE, FALSE));
+                ->will($this->onConsecutiveCalls(TRUE, TRUE, FALSE));
+
+    $this->routeBuilderIndicator->expects($this->exactly(2))
+      ->method('isRebuildStillNeeded')
+      ->will($this->onConsecutiveCalls(TRUE, FALSE));
 
     $this->yamlDiscovery->expects($this->any())
                         ->method('findAll')
@@ -293,6 +297,9 @@ public function testRebuildIfNecessary() {
 
     // This will not trigger a rebuild.
     $this->assertFalse($this->routeBuilder->rebuildIfNeeded());
+
+    // This will not trigger a rebuild, because the indicator is not set at all.
+    $this->assertFalse($this->routeBuilder->rebuildIfNeeded());
   }
 }