diff -u b/core/includes/config.inc b/core/includes/config.inc
--- b/core/includes/config.inc
+++ b/core/includes/config.inc
@@ -2,7 +2,6 @@
 
 use Drupal\Core\Config\DatabaseStorage;
 use Drupal\Core\Config\FileStorage;
-use Drupal\Core\Config\DrupalConfig;
 use Drupal\Core\Config\ConfigException;
 
 /**
@@ -81,27 +80,30 @@
  *   The name of the configuration object to retrieve. The name corresponds to
  *   a configuration file. For @code config(book.admin) @endcode, the config
  *   object returned will contain the contents of book.admin configuration file.
+ * @param $config_class
+ *   (optional) The class name for the configuration object to be returned.
+ *   Defaults to Drupal\Core\Config\DupalConfig.
  * @param $storage_class
- *   (optional) A storage driver class name to use for the DrupalConfig object.
- *   Defaults to DrupalVerifiedStorageSQL.
+ *   (optional) A storage driver class name to use for the configuration class
+ *   object. Defaults to Drupal\Core\Config\DatabaseStorage.
  *
  * @return
- *   An instance of the class specified in the $class parameter.
+ *   An instance of the $config_class class.
  *
- * @todo Replace this with an appropriate factory / ability to inject in
- *   alternate storage engines..
+ * @todo Allow to inject alternate storage engines.
  */
-function config($name, $storage_class = NULL) {
-  global $conf;
-
+function config($name, $config_class = NULL, $storage_class = NULL) {
+  if (!isset($config_class)) {
+    $config_class = 'Drupal\Core\Config\DrupalConfig';
+  }
   if (!isset($storage_class)) {
-    $storage_class = (isset($conf['config_default_storage']) ? $conf['config_default_storage'] : 'Drupal\Core\Config\DatabaseStorage');
+    $storage_class = 'Drupal\Core\Config\DatabaseStorage';
   }
-  return new DrupalConfig(new $storage_class($name));
+  return new $config_class(new $storage_class($name));
 }
 
 /**
- * Reload config from disk and write new settings to the active store.
+ * Synchronizes configuration from FileStorage to DatabaseStorage.
  */
 function config_sync() {
   $config_changes = config_get_changes_from_disk();
@@ -109,45 +111,37 @@
     return;
   }
 
-  $lock_attempts = 0;
-  $lock_acquired = FALSE;
-  do {
-    if (!$lock_acquired = lock_acquire(__FUNCTION__, 5)) {
-      lock_wait(__FUNCTION__, 5);
-      continue;
-    }
-
-    try {
-      config_sync_invoke_sync_hooks($config_changes);
-      config_sync_save_changes($config_changes);
-      // Flush all caches and reset static variables after a successful import.
-      drupal_flush_all_caches();
-    }
-    catch (ConfigException $e) {
-      watchdog_exception('config_sync', $e);
-      config_sync_invoke_sync_error_hooks($config_changes);
-      lock_release(__FUNCTION__);
-      return;
-    }
-  } while ($lock_acquired === FALSE && ++$lock_attempts < 5);
-
-  if ($lock_acquired) {
-    lock_release(__FUNCTION__);
-    return TRUE;
-  }
-  else {
-    watchdog('config_sync', 'Failed to get lock while trying to sync config.');
+  $lock_name = __FUNCTION__;
+  if (!lock_acquire($lock_name)) {
+    // Another request is synchronizing configuration. Wait for it to complete,
+    // then re-check for any remaining differences.
+    lock_wait($lock_name);
+    return config_sync();
+  }
+
+  try {
+    config_sync_invoke_sync_hooks($config_changes);
+    config_sync_save_changes($config_changes);
+    // Flush all caches and reset static variables after a successful import.
+    drupal_flush_all_caches();
+  }
+  catch (ConfigException $e) {
+    watchdog_exception('config_sync', $e);
+    config_sync_invoke_sync_error_hooks($config_changes);
+    lock_release($lock_name);
     return FALSE;
   }
+  lock_release($lock_name);
+  return TRUE;
 }
 
 /**
  * Writes an array of config file changes to the active store.
  *
- * @param $config_changes
+ * @param array $config_changes
  *   An array of changes to be written.
  */
-function config_sync_save_changes($config_changes) {
+function config_sync_save_changes(array $config_changes) {
   foreach (array('new', 'changed', 'deleted') as $type) {
     foreach ($config_changes[$type] as $name) {
       if ($type == 'deleted') {
@@ -156,7 +150,7 @@
       else {
         // Get the active store object, set the new data from file, then save.
         $target_config = config($name);
-        $source_config = config($name, 'Drupal\Core\Config\FileStorage');
+        $source_config = config($name, NULL, 'Drupal\Core\Config\FileStorage');
         $target_config->setData($source_config->get());
         $target_config->save();
       }
@@ -167,21 +161,22 @@
 /**
  * Invokes hook_config_sync_validate() and hook_config_sync() implementations.
  *
- * @param $config_changes
+ * @param array $config_changes
  *   An array of changes to be loaded.
  */
-function config_sync_invoke_sync_hooks($config_changes) {
-  // Keep a copy of the changes so that a module can't modify the values by
+function config_sync_invoke_sync_hooks(array $config_changes) {
+  // Keep a copy of the changes so that modules cannot modify the values by
   // taking the array by reference.
   $config_changes_copy = $config_changes;
 
   // @todo Lock writes to ALL config storages to prevent other/unintended config
   //   changes from happening during the import.
 
-  $target_storage = config(NULL, 'Drupal\Core\Config\DatabaseStorage');
-  $source_storage = config(NULL, 'Drupal\Core\Config\FileStorage');
+  $target_storage = config(NULL, NULL, 'Drupal\Core\Config\DatabaseStorage');
+  $source_storage = config(NULL, NULL, 'Drupal\Core\Config\FileStorage');
 
-  foreach (config_sync_sort_dependencies(module_implements('config_sync_validate')) as $module) {
+  // Allow all modules to deny configuration changes.
+  foreach (module_implements('config_sync_validate') as $module) {
     $config_changes = $config_changes_copy;
     $function = $module . '_config_sync_validate';
     $function($config_changes, $target_storage, $source_storage);
@@ -189,8 +184,12 @@
 
   // We allow modules to signal that they would like to be rerun after all
   // other modules by returning CONFIG_DEFER_SYNC. Loop until there are no
-  // modules left that indicate they would like to be rerun, checking that we're
-  // not stuck rerunning the same list of modules over and over at each cycle.
+  // modules left that indicate they would like to be re-run, checking that we
+  // are not stuck re-running the same list of modules infinitely.
+  // The list of modules is ordered by the reversed chain of module
+  // dependencies, in order to invoke dependent modules first, and thus decrease
+  // the possibility for CONFIG_DEFER_SYNC requests and increase the chance of
+  // being able to execute all changes in a single loop.
   $modules = config_sync_sort_dependencies(module_implements('config_sync'));
   do {
     // Prevent an infinite loop. If the two variables stay the same, then all
@@ -207,10 +206,9 @@
     }
   } while ($modules && $modules != $initial_module_list);
 
-  // If there are modules left that haven't run their sync hook, then we hit
-  // an infinite loop.
+  // If there are modules left, then we hit an infinite loop.
   if ($modules) {
-    throw new ConfigException("Dependency loop detected while sync configuration from disk.");
+    throw new ConfigException('Unmet dependencies detected during synchronization.');
   }
 }
 
@@ -222,15 +220,14 @@
  * made such changes, and make sure that the state of configuration in the
  * active store is correct.
  *
- * @param $config_changes
+ * @param array $config_changes
  *   An array of changes to be loaded.
  */
-function config_sync_invoke_sync_error_hooks($config_changes) {
-  $target_storage = config(NULL, 'Drupal\Core\Config\DatabaseStorage');
-  $source_storage = config(NULL, 'Drupal\Core\Config\FileStorage');
+function config_sync_invoke_sync_error_hooks(array $config_changes) {
+  $target_storage = config(NULL, NULL, 'Drupal\Core\Config\DatabaseStorage');
+  $source_storage = config(NULL, NULL, 'Drupal\Core\Config\FileStorage');
 
-  $modules = config_sync_sort_dependencies(module_implements('config_sync_error'));
-  foreach ($modules as $module) {
+  foreach (module_implements('config_sync_error') as $module) {
     $function = $module . '_config_sync_error';
     try {
       $function($config_changes, $target_storage, $source_storage);
@@ -243,15 +240,16 @@
 }
 
 /**
- * Sort the given list of modules based on dependency.
+ * Sorts a given list of modules based on their dependencies.
  *
- * @param $modules
+ * @param array $modules
  *   A list of modules.
- * @return
+ *
+ * @return array
  *   The list of modules sorted by dependency.
  */
-function config_sync_sort_dependencies($modules) {
-  // Get all module data so we can find find weights and sort.
+function config_sync_sort_dependencies(array $modules) {
+  // Get all module data so we can find weights and sort.
   $module_data = system_rebuild_module_data();
 
   $sorted_modules = array();
@@ -265,8 +263,9 @@
 /**
  * Returns a list of changes on disk compared to the active store.
  *
- * @return
- *   The list of files changed on disk compared to the active store.
+ * @return array|bool
+ *   The list of files changed on disk compared to the active store, or FALSE if
+ *   there are no differences.
  */
 function config_get_changes_from_disk() {
   $disk_config_names = FileStorage::getNamesWithPrefix();
@@ -278,14 +277,18 @@
   );
   foreach (array_intersect($disk_config_names, $active_config_names) as $name) {
     $active_config = config($name);
-    $file_config = config($name, 'Drupal\Core\Config\FileStorage');
+    $file_config = config($name, NULL, 'Drupal\Core\Config\FileStorage');
     if ($active_config->get() != $file_config->get()) {
       $config_changes['changed'][] = $name;
     }
   }
+
+  // Do not trigger subsequent synchronization operations if there are no
+  // changes in either category.
   if (empty($config_changes['new']) && empty($config_changes['changed']) && empty($config_changes['deleted'])) {
     return FALSE;
   }
+
   return $config_changes;
 }
 
diff -u b/core/modules/config/config.api.php b/core/modules/config/config.api.php
--- b/core/modules/config/config.api.php
+++ b/core/modules/config/config.api.php
@@ -3,33 +3,123 @@
 /**
  * @file
- * Hooks provided by the Config module.
+ * Hooks provided by the Configuration module.
  */
 
 /**
  * @defgroup config_hooks Configuration system hooks
  * @{
- * TODO: overall description of the configuration system.
+ * @todo Overall description of the configuration system.
  * @}
  */
 
 /**
- * Reload config changes from disk.
+ * Validate configuration changes before they are synchronized.
+ *
+ * @param array $config_changes
+ *   An associative array whose keys denote the configuration differences
+ *   ('new', 'changed', 'deleted') and whose values are arrays of configuration
+ *   object names.
+ * @param $target_storage
+ *   A configuration class acting on the target storage to which the new
+ *   configuration will be written. Use $target_storage->load() to load a
+ *   configuration object.
+ * @param $source_storage
+ *   A configuration class acting on the source storage from which configuration
+ *   differences were read. Use $target_storage->load() to load a configuration
+ *   object.
+ *
+ * @throws ConfigException
+ *   In case a configuration change cannot be allowed.
  */
-function hook_config_sync($config_changes, $target_storage, $source_storage) {
-  // TODO: working example.
+function hook_config_sync_validate($config_changes, $target_storage, $source_storage) {
+  // Deny changes to our settings.
+  if (isset($config_changes['changed']['mymodule.locked'])) {
+    throw new ConfigException('MyModule settings cannot be changed.');
+  }
 }
 
 /**
- * Validate configuration changes before they are saved to the active store.
+ * Synchronize configuration changes.
+ *
+ * This hook is invoked when configuration is synchronized between storages and
+ * allows all modules to react to new, deleted, and changed configuration. This
+ * hook is invoked before the new configuration is written to the target storage.
+ * Implementations of this hook only react to the differences. The new
+ * configuration itself is written by the configuration system.
+ *
+ * @param array $config_changes
+ *   An associative array whose keys denote the configuration differences
+ *   ('new', 'changed', 'deleted') and whose values are arrays of configuration
+ *   object names.
+ * @param $target_storage
+ *   A configuration class acting on the target storage to which the new
+ *   configuration will be written. Use $target_storage->load() to load a
+ *   configuration object.
+ * @param $source_storage
+ *   A configuration class acting on the source storage from which configuration
+ *   differences were read. Use $target_storage->load() to load a configuration
+ *   object.
+ *
+ * @return
+ *   Nothing on successful synchronization, or CONFIG_DEFER_SYNC if a change
+ *   cannot be synchronized yet adn depends on another module to execute first.
  */
-function hook_config_sync_validate($config_changes, $target_storage, $source_storage) {
-  // TODO: working example.
+function hook_config_sync($config_changes, $target_storage, $source_storage) {
+  foreach ($config_changes['new'] as $name) {
+    if (strpos($name, 'image.style.') === 0) {
+      // Load the new configuration data and inform other modules about it.
+      $style = $source_storage->load($name)->get();
+      $style['is_new'] = TRUE;
+      module_invoke_all('image_style_save', $style);
+    }
+  }
+  foreach ($config_changes['changed'] as $name) {
+    if (strpos($name, 'image.style.') === 0) {
+      // Load the new configuration data, inform other modules about the change,
+      // and perform any further actions that may be required for the change to
+      // take effect.
+      $style = $source_storage->load($name)->get();
+      $style['is_new'] = FALSE;
+      module_invoke_all('image_style_save', $style);
+      // Delete existing derivative images.
+      image_style_flush($style);
+    }
+  }
+  foreach ($config_changes['deleted'] as $name) {
+    if (strpos($name, 'image.style.') === 0) {
+      // The style has been deleted, so read the previous configuration from the
+      // old storage.
+      $style = $target_storage->load($name)->get();
+      module_invoke_all('image_style_delete', $style);
+      // Delete existing derivative images.
+      image_style_flush($style);
+    }
+  }
 }
 
 /**
  * Validate configuration changes before they are saved to the active store.
+ *
+ * During synchronization of configuration, modules may make changes that cannot
+ * be rolled back. This hook allows modules to react to an error that occurs
+ * after they have made such changes, and make sure that the state of
+ * configuration is as correct as possible.
+ *
+ * @param array $config_changes
+ *   An associative array whose keys denote the configuration differences
+ *   ('new', 'changed', 'deleted') and whose values are arrays of configuration
+ *   object names.
+ * @param $target_storage
+ *   A configuration class acting on the target storage to which the new
+ *   configuration will be written. Use $target_storage->load() to load a
+ *   configuration object.
+ * @param $source_storage
+ *   A configuration class acting on the source storage from which configuration
+ *   differences were read. Use $target_storage->load() to load a configuration
+ *   object.
  */
 function hook_config_sync_error($config_changes, $target_storage, $source_storage) {
-  // TODO: working example.
+  // @todo Feasability and usage of this hook is still unclear, without having a
+  //   backup of $target_storage at hand.
 }