diff --git a/imagecache.module b/imagecache.module
index 55d48ce..8cc9752 100644
--- a/imagecache.module
+++ b/imagecache.module
@@ -442,7 +442,7 @@ function _imagecache_cache($presetname, $path) {
   }
 
   // umm yeah deliver it early if it is there. especially useful
-  // to prevent lock files from being created when delivering private files.
+  // to prevent locks from being created when delivering private files.
   $dst = imagecache_create_path($preset['presetname'], $path);
   if (is_file($dst)) {
     imagecache_transfer($dst);
@@ -458,30 +458,26 @@ function _imagecache_cache($presetname, $path) {
     exit;
   };
 
-  // Bail if the requested file isn't an image you can't request .php files
+  // Bail if the requested file isn't an image. You can't request .php files
   // etc...
-  if (!getimagesize($src)) {
+  if (!@getimagesize($src)) {
     watchdog('imagecache', '403: File is not an image %image ', array('%image' => $src), WATCHDOG_ERROR);
     header('HTTP/1.0 403 Forbidden');
     exit;
   }
 
-  $lockfile = file_directory_temp() .'/'. $preset['presetname'] . basename($src);
-  if (file_exists($lockfile)) {
-    watchdog('imagecache', 'ImageCache already generating: %dst, Lock file: %tmp.', array('%dst' => $dst, '%tmp' => $lockfile), WATCHDOG_NOTICE);
-    // 307 Temporary Redirect, to myself. Lets hope the image is done next time around.
-    header('Location: '. request_uri(), TRUE, 307);
-    exit;
+  // Generate preset inside of a lock.
+  $lockname = $preset['presetname'] . basename($src);
+  if (lock_acquire($lockname)) {
+    imagecache_build_derivative($preset['actions'], $src, $dst);
+  }
+  else {
+    lock_wait($lockname);
   }
-  touch($lockfile);
-  // register the shtdown function to clean up lock files. by the time shutdown
-  // functions are being called the cwd has changed from document root, to
-  // server root so absolute paths must be used for files in shutdown functions.
-  register_shutdown_function('file_delete', realpath($lockfile));
-
-  // check if deriv exists... (file was created between apaches request handler and reaching this code)
-  // otherwise try to create the derivative.
-  if (file_exists($dst) || imagecache_build_derivative($preset['actions'], $src, $dst)) {
+
+  // Make sure derivative image exists before trying to send it.
+  if (file_exists($dst)) {
+    // exit gets called inside this function.
     imagecache_transfer($dst);
   }
   // Generate an error if image could not generate.