switching to a less-blocking COW list fixes mutex

seems to resolve GH-5520

core/src/main/java/org/jruby/RubyThread.java

@@ -50,10 +50,7 @@
 import java.util.ArrayList;
 import java.util.Map;
 import java.util.Set;
-import java.util.concurrent.ConcurrentLinkedQueue;
-import java.util.concurrent.ExecutionException;
-import java.util.concurrent.Semaphore;
-import java.util.concurrent.TimeUnit;
+import java.util.concurrent.*;
 import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.concurrent.locks.Lock;
@@ -161,7 +158,7 @@ public class RubyThread extends RubyObject implements ExecutionContext {
     //private volatile boolean handleInterrupt = true;
 
     /** Stack of interrupt masks active for this thread */
-    private final List<RubyHash> interruptMaskStack = Collections.synchronizedList(new ArrayList<RubyHash>());
+    private final List<RubyHash> interruptMaskStack = new CopyOnWriteArrayList<>();
 
     /** Thread-local tuple used for sleeping (semaphore, millis, nanos) */
     private final SleepTask2 sleepTask = new SleepTask2();
@@ -345,12 +342,7 @@ private int pendingInterruptCheckMask(ThreadContext context, IRubyObject err) {
         List<IRubyObject> ancestors = getMetaClass(err).getAncestorList();
         int ancestorsLen = ancestors.size();
 
-        List<RubyHash> maskStack = interruptMaskStack;
-        int maskStackLen = maskStack.size();
-
-        for (int i = 0; i < maskStackLen; i++) {
-            RubyHash mask = maskStack.get(maskStackLen - (i + 1));
-
+        for (RubyHash mask : interruptMaskStack) {
             for (int j = 0; j < ancestorsLen; j++) {
                 IRubyObject klass = ancestors.get(j);
                 IRubyObject sym;