Potential race condition in BaseOperationImpl.java

[MusaTalluzi]

Our profiler shows BaseOperationImpl.isCancelled spends 90% of the time in blocked state and we suspect it's causing deadlocks since it shares the same lock with other operations in the Operation class:

  public final synchronized boolean isCancelled() {
    return cancelled;
  }

  public final synchronized void cancel() {
    cancelled = true;

    synchronized (clones) {
      Iterator<Operation> i = clones.iterator();
      while(i.hasNext()) {
        i.next().cancel();
      }
    }

    wasCancelled();
    callback.receivedStatus(CANCELLED);
    callback.complete();
  }

If the operation is cloned (original X, clone Y) and two threads call cancel():

• Thread A calls X.cancel(), aquires X's lock and yields
• Thread B calls Y.cancel(), aquires Y's lock, enters synchronized block, i.next() returns X, thread blocks on i.next().cancel() since thread A is already executing that function
• Thread A resumes execution, its i.next() returns A and it blocks similar to B

Is this a valid situation that could happen?