Re-acquire locks via iterative instead of recursive execution #447

When multiple OrderedLocks are acquired by different threads, the
deadlock recovery mechanism suspending and reacquiring the locks
requires an indefinite number of tries until one thread holds all
required locks. Since reacquisition is performed by recursive method
invocation, the stack can become infinitely large with the change of
resulting in a StackOverflowError.

These change replace the recursive lock acqusition by an iterative one,
such that still an indefinite number of tries for acquiring a set of
locks is required but the chance of resulting in an error is eliminated.

The added test case does not deterministically reproduce the erroneous
behavior, but since a proper regression test is very hard to define (as
specific lock order across a magnitude of retries has to be ensured and
coordinated between different threads), it at least executes more
sophisticated locking scenarios to ensure proper lock retrieval and
deadlock management.

runtime/bundles/org.eclipse.core.jobs/src/org/eclipse/core/internal/jobs/LockManager.java

@@ -49,19 +49,16 @@ protected static LockState suspend(OrderedLock lock) {
 		/**
 		 * Re-acquires a suspended lock and reverts to the correct lock depth.
 		 */
-		public void resume() {
-			//spin until the lock is successfully acquired
-			//NOTE: spinning here allows the UI thread to service pending syncExecs
-			//if the UI thread is waiting to acquire a lock.
-			while (true) {
-				try {
-					if (lock.acquire(Long.MAX_VALUE))
-						break;
-				} catch (InterruptedException e) {
-					//ignore and loop
+		public boolean resume() {
+			try {
+				if (lock.acquire(Integer.MAX_VALUE, false)) {
+					lock.setDepth(depth);
+					return true;
 				}
+			} catch (InterruptedException e) {
+				// ignore and retry
 			}
-			lock.setDepth(depth);
+			return false;
 		}
 	}
 
@@ -303,17 +300,26 @@ void removeLockWaitThread(Thread thread, ISchedulingRule lock) {
 	 * Resumes all the locks that were suspended while this thread was waiting to acquire another lock.
 	 */
 	void resumeSuspendedLocks(Thread owner) {
-		LockState[] toResume;
-		synchronized (suspendedLocks) {
-			Deque<LockState[]> prevLocks = suspendedLocks.get(owner);
-			if (prevLocks == null)
-				return;
-			toResume = prevLocks.pop();
-			if (prevLocks.isEmpty())
-				suspendedLocks.remove(owner);
+		// spin until the locks are successfully acquired
+		// NOTE: spinning here allows the UI thread to service pending syncExecs
+		// if the UI thread is waiting to acquire a lock.
+		while (true) {
+			LockState[] toResume;
+			synchronized (suspendedLocks) {
+				Deque<LockState[]> prevLocks = suspendedLocks.get(owner);
+				if (prevLocks == null)
+					return;
+				toResume = prevLocks.pop();
+				if (prevLocks.isEmpty())
+					suspendedLocks.remove(owner);
+			}
+			for (LockState element : toResume) {
+				if (!element.resume()) {
+					// Resuming a lock failed, so start with the first one again.
+					break;
+				}
+			}
 		}
-		for (LockState element : toResume)
-			element.resume();
 	}
 
 	public void setLockListener(LockListener listener) {

runtime/bundles/org.eclipse.core.jobs/src/org/eclipse/core/internal/jobs/OrderedLock.java

@@ -97,6 +97,21 @@ public void acquire() {
 
 	@Override
 	public boolean acquire(long delay) throws InterruptedException {
+		return acquire(delay, true);
+	}
+
+	/**
+	 * Acquires a lock as defined by {@link #acquire(long)} but only resumes
+	 * suspended locks if {@code resumeSuspendedLocks} is {@code true}.
+	 *
+	 * @param delay                the number of milliseconds to delay
+	 * @param resumeSuspendedLocks whether locks that were suspended because a lock
+	 *                             could not be acquired shall be re-acquired or not
+	 * @return {@code true} if the lock was successfully acquired, and {@code false}
+	 *         otherwise.
+	 * @exception InterruptedException if the thread was interrupted
+	 */
+	boolean acquire(long delay, boolean resumeSuspendedLocks) throws InterruptedException {
 		if (Thread.interrupted())
 			throw new InterruptedException();
 
@@ -108,7 +123,9 @@ public boolean acquire(long delay) throws InterruptedException {
 		if (DEBUG)
 			System.out.println("[" + Thread.currentThread() + "] Operation waiting to be executed... " + this); //$NON-NLS-1$ //$NON-NLS-2$
 		boolean success = doAcquire(semaphore, delay);
-		manager.resumeSuspendedLocks(Thread.currentThread());
+		if (resumeSuspendedLocks) {
+			manager.resumeSuspendedLocks(Thread.currentThread());
+		}
 		if (DEBUG)
 			System.out.println("[" + Thread.currentThread() + //$NON-NLS-1$
 					(success ? "] Operation started... " : "] Operation timed out... ") + this); //$NON-NLS-1$ //$NON-NLS-2$ //}

runtime/tests/org.eclipse.core.tests.runtime/src/org/eclipse/core/tests/runtime/jobs/OrderedLockTest.java

@@ -63,6 +63,37 @@ public void testComplex() {
 		});
 	}
 
+	@Test
+	public void testManyLocksAndThreads() {
+		int numberOfLocks = 30;
+		int numberOfThreads = 15;
+		DeadlockDetector.runSilent(() -> {
+			ArrayList<LockAcquiringRunnable> allRunnables = new ArrayList<>();
+			LockManager manager = new LockManager();
+			manager.setLockListener(new LockListener() {
+				@Override
+				public boolean aboutToWait(Thread lockOwner) {
+					// Yield upon waiting for a lock to give other threads the change for
+					// conflicting lock acquisitions
+					Thread.yield();
+					return false;
+				}
+			});
+			List<OrderedLock> locks = new ArrayList<>();
+			for (int i = 0; i < numberOfLocks; i++) {
+				locks.add(manager.newLock());
+			}
+			for (int i = 0; i < numberOfThreads / 5; i++) {
+				Collections.shuffle(locks);
+				// Assign random half of the locks to every thread
+				createRunnables(locks.stream().limit(numberOfLocks / 2).toArray(OrderedLock[]::new), 5, allRunnables);
+			}
+			execute(allRunnables);
+			// the underlying array has to be empty
+			assertTrue("Locks not removed from graph.", manager.isEmpty());
+		});
+	}
+
 	@Test
 	public void testSimple() {
 		DeadlockDetector.runSilent(() -> {