Bug 296822 NullProgressMonitor is not thread safe for expected usage
NullProgressMonitor.setCanceled and isCanceled access the cancelled field, but there's no synchronization. The expected use is that setCanceled is called from a thread other than the one doing the work, which polls isCanceled(), so this needs to be thread safe. Could either make these two methods synchronized or make the field volatile. Should check other progress monitor implementations too.
bundles/org.eclipse.jface/src/org/eclipse/jface/dialogs/ProgressMonitorDialog.java
locked = true;
			updateForSetBlocked(reason);
		}
	}
	/**
	 * Clear blocked state from the receiver.
	 */
	protected void updateForClearBlocked() {
		progressIndicator.showNormal();
		setMessage(task, true);
		imageLabel.setImage(getImage());
		
	}
	/**
	 * Set blocked state from the receiver.
	 * 
	 * @param reason
	 *            IStatus that gives the details
	 */
	protected void updateForSetBlocked(IStatus reason) {
		progressIndicator.showPaused();
		setMessage(reason.getMessage(), true);
		imageLabel.setImage(getImage());
		
	}
	/**
	 * Creates a progress monitor dialog under the given shell. The dialog has a
	 * standard title and no image. <code>open</code> is non-blocking.
	 * 
	 * @param parent