gistfile1

diff --git a/src/org/jruby/ext/fiber/Fiber.java b/src/org/jruby/ext/fiber/Fiber.java
index 8731a38..70c513f 100644
--- a/src/org/jruby/ext/fiber/Fiber.java
+++ b/src/org/jruby/ext/fiber/Fiber.java
@@ -20,6 +20,7 @@ public abstract class Fiber extends RubyObject implements ExecutionContext {
     private final Map<Object, IRubyObject> contextVariables = new WeakHashMap<Object, IRubyObject>();
     protected volatile Block block;
     protected volatile RubyThread parent;
+    protected volatile RubyThread master;
     protected boolean root;
     protected volatile Fiber transferredFrom;
     protected volatile Fiber transferredTo;
@@ -32,6 +33,9 @@ public abstract class Fiber extends RubyObject implements ExecutionContext {
         }
         this.block = block;
         this.parent = context.getThread();
+
+        // store master thread, so we know when we're resumed from others
+        master = getMasterFrom(context);
         
         initFiber(context);
         
@@ -61,34 +65,46 @@ public abstract class Fiber extends RubyObject implements ExecutionContext {
 
     @JRubyMethod()
     public IRubyObject resume(ThreadContext context) {
+        checkMasterThread(context);
+
         return resumeOrTransfer(context, context.nil, false);
     }
 
     @JRubyMethod()
     public IRubyObject resume(ThreadContext context, IRubyObject arg) {
+        checkMasterThread(context);
+
         return resumeOrTransfer(context, arg, false);
     }
 
     @JRubyMethod(rest = true)
     public IRubyObject resume(ThreadContext context, IRubyObject[] args) {
+        checkMasterThread(context);
+
         return resumeOrTransfer(context, context.getRuntime().newArrayNoCopyLight(args), false);
     }
 
     // This should only be defined after require 'fiber'
     @JRubyMethod()
     public IRubyObject transfer(ThreadContext context) {
+        checkMasterThread(context);
+
         return resumeOrTransfer(context, context.nil, true);
     }
 
     // This should only be defined after require 'fiber'
     @JRubyMethod()
     public IRubyObject transfer(ThreadContext context, IRubyObject arg) {
+        checkMasterThread(context);
+
         return resumeOrTransfer(context, arg, true);
     }
 
     // This should only be defined after require 'fiber'
     @JRubyMethod(rest = true)
     public IRubyObject transfer(ThreadContext context, IRubyObject[] args) {
+        checkMasterThread(context);
+
         return resumeOrTransfer(context, context.getRuntime().newArrayNoCopyLight(args), true);
     }
 
@@ -123,5 +139,25 @@ public abstract class Fiber extends RubyObject implements ExecutionContext {
     public void setTransferredTo(Fiber transferredTo) {
         this.transferredTo = transferredTo;
     }
-    
+
+    /**
+     * @return the RubyThread that owns this fiber
+     */
+    public RubyThread getMaster() {
+        return master;
+    }
+
+    private void checkMasterThread(ThreadContext context) {
+        if (master != getMasterFrom(context)) {
+            throw context.runtime.newFiberError("fiber called across threads");
+        }
+    }
+
+    private static RubyThread getMasterFrom(ThreadContext context) {
+        if (context.getFiber() != null) {
+            return context.getFiber().getMaster();
+        } else {
+            return context.getThread();
+        }
+    }
 }