diff --git a/core/includes/update.inc b/core/includes/update.inc
index 5582bb4..6794ca7 100644
--- a/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -578,6 +578,11 @@ function update_batch($start, $redirect = NULL, $url = NULL, $batch = array(), $
  * @see update_batch()
  */
 function update_finished($success, $results, $operations) {
+  // Unlock the module system to flush all caches.
+  module_list_reset();
+  // Load all modules, so data structures can be properly rebuilt.
+  module_load_all(FALSE, TRUE);
+
   // Clear the caches in case the data has been updated.
   drupal_flush_all_caches();
 
diff --git a/core/update.php b/core/update.php
index 1a00777..35b42b4 100644
--- a/core/update.php
+++ b/core/update.php
@@ -129,6 +129,11 @@ function update_script_selection_form($form, &$form_state) {
       '#links' => update_helpful_links(),
     );
 
+    // Unlock the module system to flush all caches.
+    module_list_reset();
+    // Load all modules, so data structures can be properly rebuilt.
+    module_load_all(FALSE, TRUE);
+
     // No updates to run, so caches won't get flushed later.  Clear them now.
     drupal_flush_all_caches();
   }
@@ -375,9 +380,20 @@ function update_check_requirements($skip_warnings = FALSE) {
 require_once DRUPAL_ROOT . '/core/includes/unicode.inc';
 require_once DRUPAL_ROOT . '/core/includes/schema.inc';
 
-// Limit the module list to System module for the entire duration of update.php.
+// The module list must be limited to System module for almost all parts of the
+// update process to prevent module APIs and hooks from being invoked too early
+// (trying to act on data structures that are not updated yet).
+// This especially applies to early bootstrap preparations when upgrading to a
+// new major version of Drupal core. Unfortunately, however, the initial
+// information and update selection pages (and forms) need to be able to invoke
+// hook_requirements() in all modules and also access other APIs. Therefore, the
+// effective module list has to be switched back and forth multiple times during
+// the update.php process.
 include_once DRUPAL_ROOT . '/core/includes/module.inc';
 $module_list['system']['filename'] = 'core/modules/system/system.module';
+
+// Lock the module list for the update bootstrap and performing initial system
+// changes.
 module_list(NULL, $module_list);
 
 update_prepare_d8_bootstrap();
@@ -418,6 +434,9 @@ function update_check_requirements($skip_warnings = FALSE) {
   // Set up theme system for the maintenance page.
   drupal_maintenance_theme();
 
+  // Unlock the module system to perform update requirements checks.
+  module_list_reset();
+
   // Check the update requirements for Drupal. Only report on errors at this
   // stage, since the real requirements check happens further down.
   update_check_requirements(TRUE);
@@ -447,6 +466,9 @@ function update_check_requirements($skip_warnings = FALSE) {
 
   update_fix_compatibility();
 
+  // Unlock the module system to perform update requirements checks.
+  module_list_reset();
+
   // Check the update requirements for all modules. If there are warnings, but
   // no errors, skip reporting them if the user has provided a URL parameter
   // acknowledging the warnings and indicating a desire to continue anyway. See
@@ -455,6 +477,9 @@ function update_check_requirements($skip_warnings = FALSE) {
   $skip_warnings = !empty($continue);
   update_check_requirements($skip_warnings);
 
+  // Re-lock the module system.
+  module_list(NULL, $module_list);
+
   switch ($op) {
     // update.php ops.
 
@@ -482,6 +507,11 @@ function update_check_requirements($skip_warnings = FALSE) {
       break;
 
     case 'results':
+      // Unlock the module system after performing updates.
+      module_list_reset();
+      // Load all modules to flush all caches and rebuild data structures.
+      module_load_all(FALSE, TRUE);
+
       $output = update_results_page();
       break;