gistfile1.txt

diff --git a/src/org/jruby/runtime/load/LoadService.java b/src/org/jruby/runtime/load/LoadService.java
index 12b3f83..506dd72 100644
--- a/src/org/jruby/runtime/load/LoadService.java
+++ b/src/org/jruby/runtime/load/LoadService.java
@@ -205,6 +205,8 @@ public class LoadService {
     protected final Map<String, JarFile> jarFiles = new HashMap<String, JarFile>();
 
     protected final Ruby runtime;
+    
+    protected final ReentrantLock requireToken = new ReentrantLock();
 
     public LoadService(Ruby runtime) {
         this.runtime = runtime;
@@ -329,7 +331,17 @@ public class LoadService {
     }
 
     public boolean require(String requireName) {
-        return requireCommon(requireName, true) == RequireState.LOADED;
+        boolean success = requireToken.tryLock();
+        if (!success) {
+            runtime.getWarnings().warn("concurrent require of `" + requireName + "` detected");
+        }
+        try {
+            return requireCommon(requireName, true) == RequireState.LOADED;
+        } finally {
+            if (success) {
+                requireToken.unlock();
+            }
+        }
     }
 
     public boolean autoloadRequire(String requireName) {