diff -bduNr jsonlog/jsonlog.module jsonlog.b/jsonlog.module
--- jsonlog/jsonlog.module	2014-10-21 20:10:06.000000000 +0200
+++ jsonlog.b/jsonlog.module	2014-11-11 09:23:13.488485677 +0100
@@ -611,7 +611,7 @@
 
 
   // File append. If failure: log filing error to web server's default log.
-  if (!file_put_contents($_file, "\n" . drupal_json_encode($entry), FILE_APPEND)) {
+  if (!file_put_contents($_file, "\n" . drupal_json_encode($entry), FILE_APPEND | LOCK_EX )) {
     error_log('Drupal jsonlog, site ID[' . $_site_id . '], failed to write to file[' . $_file . '].');
   }
 }