Mercurial > hg > Members > kono > jpf-core
view src/examples/oldclassic.java @ 0:61d41facf527
initial v8 import (history reset)
author | Peter Mehlitz <Peter.C.Mehlitz@nasa.gov> |
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date | Fri, 23 Jan 2015 10:14:01 -0800 |
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/* * Copyright (C) 2014, United States Government, as represented by the * Administrator of the National Aeronautics and Space Administration. * All rights reserved. * * The Java Pathfinder core (jpf-core) platform is licensed under the * Apache License, Version 2.0 (the "License"); you may not use this file except * in compliance with the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0. * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ /** * This example shows a deadlock that occurs as a result of a missed signal, * i.e. a wait() that happens after the corresponding notify(). * * The defect is caused by a violated monitor encapsulation, i.e. directly * accessing monitor internal data ('Event.count') from concurrent clients * ('FirstTask', 'SecondTask'), without synchronization with the * corresponding monitor operations ('wait_for-Event()' and 'signalEvent()'). * * The resulting race is typical for unsafe optimizations that try to * avoid expensive blocking calls by means of local caches * * This example was inspired by a defect found in the "Remote Agent" * spacecraft controller that flew on board of "Deep Space 1", as described * in: * * Model Checking Programs * W. Visser, K. Havelund, G. Brat, S. Park and F. Lerda * Automated Software Engineering Journal * Volume 10, Number 2, April 2003 * * @author wvisser */ //------- the test driver public class oldclassic { public static void main (String[] args) { Event new_event1 = new Event(); Event new_event2 = new Event(); FirstTask task1 = new FirstTask(new_event1, new_event2); SecondTask task2 = new SecondTask(new_event1, new_event2); task1.start(); task2.start(); } } //------- shared objects implemented as monitors class Event { int count = 0; public synchronized void signal_event () { // NOTE: this abstraction is not strictly required - even if the state space would // be unbound, JPF could still find the error at a reasonable search depth, // unless it's left-most branch in the search tree is unbound. If it is, // there are two ways to work around: (1) use a different search strategy // (e.g. HeuristicSearch with BFSHeuristic), or (2) set a random choice // enumeration order ("+cg.randomize_choices=true"). In this example, (2) // works just fine count = (count + 1) % 3; //count++; // requires "+cg.randomize_choices=true" for DFSearch policy notifyAll(); } public synchronized void wait_for_event () { try { wait(); } catch (InterruptedException e) { } } } //------- the two concurrent threads using the monitors class FirstTask extends Thread { Event event1; Event event2; int count = 0; // bad optimization - local cache of event1 internals public FirstTask (Event e1, Event e2) { this.event1 = e1; this.event2 = e2; } @Override public void run () { count = event1.count; // <race> violates event1 monitor encapsulation while (true) { System.out.println("1"); if (count == event1.count) { // <race> ditto event1.wait_for_event(); } count = event1.count; // <race> ditto event2.signal_event(); // updates event2.count } } } class SecondTask extends Thread { Event event1; Event event2; int count = 0; // bad optimization - local cache of event2 internals public SecondTask (Event e1, Event e2) { this.event1 = e1; this.event2 = e2; } @Override public void run () { count = event2.count; // <race> violates event2 monitor encapsulation while (true) { System.out.println(" 2"); event1.signal_event(); // updates event1.count if (count == event2.count) { // <race> ditto event2.wait_for_event(); } count = event2.count; // <race> ditto } } }