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8345052: Harden StampedLock
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Reviewed-by: alanb, vklang
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Doug Lea committed Nov 29, 2024
1 parent 095e769 commit ece0401
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Showing 2 changed files with 250 additions and 50 deletions.
158 changes: 108 additions & 50 deletions src/java.base/share/classes/java/util/concurrent/locks/StampedLock.java
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Original file line number Diff line number Diff line change
Expand Up @@ -289,7 +289,8 @@ public class StampedLock implements java.io.Serializable {
* Nearly all of these mechanics are carried out in methods
* acquireWrite and acquireRead, that, as typical of such code,
* sprawl out because actions and retries rely on consistent sets
* of locally cached reads.
* of locally cached reads, and include fall-backs for exceptional
* cases including OutOfMemoryErrors and JVM exceptions.
*
* For an explanation of the use of acquireFence, see
* http://gee.cs.oswego.edu/dl/html/j9mm.html as well as Boehm's
Expand Down Expand Up @@ -1182,8 +1183,7 @@ private boolean casTail(Node c, Node v) {
}

/** tries once to CAS a new dummy node for head */
private void tryInitializeHead() {
Node h = new WriterNode();
private void tryInitializeHead(Node h) {
if (U.compareAndSetReference(this, HEAD, null, h))
tail = h;
}
Expand All @@ -1203,6 +1203,7 @@ private long acquireWrite(boolean interruptible, boolean timed, long time) {
boolean interrupted = false, first = false;
WriterNode node = null;
Node pred = null;
long nanos = 0L;
for (long s, nextState;;) {
if (!first && (pred = (node == null) ? null : node.prev) != null &&
!(first = (head == pred))) {
Expand All @@ -1227,12 +1228,23 @@ private long acquireWrite(boolean interruptible, boolean timed, long time) {
}
return nextState;
} else if (node == null) { // retry before enqueuing
node = new WriterNode();
try {
node = new WriterNode();
} catch (OutOfMemoryError oome) {
return spinLockOnOOME(true, interruptible, timed, time);
}
} else if (pred == null) { // try to enqueue
Node t = tail;
node.setPrevRelaxed(t);
if (t == null)
tryInitializeHead();
if (t == null) { // try to initialize
Node h;
try {
h = new WriterNode();
} catch (OutOfMemoryError oome) {
return spinLockOnOOME(true, interruptible, timed, time);
}
tryInitializeHead(h);
}
else if (!casTail(t, node))
node.setPrevRelaxed(null); // back out
else
Expand All @@ -1244,21 +1256,25 @@ else if (!casTail(t, node))
if (node.waiter == null)
node.waiter = Thread.currentThread();
node.status = WAITING;
} else {
long nanos;
spins = postSpins = (byte)((postSpins << 1) | 1);
if (!timed)
LockSupport.park(this);
else if ((nanos = time - System.nanoTime()) > 0L)
LockSupport.parkNanos(this, nanos);
else
break;
} else if (!timed || (nanos = time - System.nanoTime()) > 0L) {
try {
if (!timed)
LockSupport.park(this);
else
LockSupport.parkNanos(this, nanos);
} catch (Error | RuntimeException ex) {
cancelAcquire(node);
throw ex;
}
node.clearStatus();
if ((interrupted |= Thread.interrupted()) && interruptible)
break;
}
spins = postSpins = (byte)((postSpins << 1) | 1);
} else
break;
}
return cancelAcquire(node, interrupted);
cancelAcquire(node);
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
}

/**
Expand All @@ -1285,11 +1301,23 @@ private long acquireRead(boolean interruptible, boolean timed, long time) {
if ((t == null || (tailPred = t.prev) == null) &&
(nextState = tryAcquireRead()) != 0L) // try now if empty
return nextState;
else if (t == null)
tryInitializeHead();
else if (t == null) {
Node h;
try {
h = new WriterNode();
} catch (OutOfMemoryError oome) {
return spinLockOnOOME(false, interruptible, timed, time);
}
tryInitializeHead(h);
}
else if (tailPred == null || !(t instanceof ReaderNode)) {
if (node == null)
node = new ReaderNode();
if (node == null) {
try {
node = new ReaderNode();
} catch (OutOfMemoryError oome) {
return spinLockOnOOME(false, interruptible, timed, time);
}
}
if (tail == t) {
node.setPrevRelaxed(t);
if (casTail(t, node)) {
Expand All @@ -1302,8 +1330,13 @@ else if (tailPred == null || !(t instanceof ReaderNode)) {
for (boolean attached = false;;) {
if (leader.status < 0 || leader.prev == null)
break;
else if (node == null)
node = new ReaderNode();
else if (node == null) {
try {
node = new ReaderNode();
} catch (OutOfMemoryError oome) {
return spinLockOnOOME(false, interruptible, timed, time);
}
}
else if (node.waiter == null)
node.waiter = Thread.currentThread();
else if (!attached) {
Expand All @@ -1313,15 +1346,22 @@ else if (!attached) {
if (!attached)
node.setCowaitersRelaxed(null);
} else {
long nanos = 0L;
if (!timed)
LockSupport.park(this);
else if ((nanos = time - System.nanoTime()) > 0L)
LockSupport.parkNanos(this, nanos);
long nanos = (timed) ? time - System.nanoTime(): 0L;
try {
if (!timed)
LockSupport.park(this);
else if (nanos > 0L)
LockSupport.parkNanos(this, nanos);
} catch (Error | RuntimeException ex) {
cancelCowaiter(node, leader);
throw ex;
}
interrupted |= Thread.interrupted();
if ((interrupted && interruptible) ||
(timed && nanos <= 0L))
return cancelCowaiter(node, leader, interrupted);
(timed && nanos <= 0L)) {
cancelCowaiter(node, leader);
return (interrupted) ? INTERRUPTED : 0L;
}
}
}
if (node != null)
Expand All @@ -1341,6 +1381,7 @@ else if ((nanos = time - System.nanoTime()) > 0L)
byte spins = 0, postSpins = 0; // retries upon unpark of first thread
boolean first = false;
Node pred = null;
long nanos = 0L;
for (long nextState;;) {
if (!first && (pred = node.prev) != null &&
!(first = (head == pred))) {
Expand Down Expand Up @@ -1371,21 +1412,25 @@ else if ((nanos = time - System.nanoTime()) > 0L)
if (node.waiter == null)
node.waiter = Thread.currentThread();
node.status = WAITING;
} else {
long nanos;
spins = postSpins = (byte)((postSpins << 1) | 1);
if (!timed)
LockSupport.park(this);
else if ((nanos = time - System.nanoTime()) > 0L)
LockSupport.parkNanos(this, nanos);
else
break;
} else if (!timed || (nanos = time - System.nanoTime()) > 0) {
try {
if (!timed)
LockSupport.park(this);
else
LockSupport.parkNanos(this, nanos);
} catch (Error | RuntimeException ex) {
cancelAcquire(node);
throw ex;
}
node.clearStatus();
if ((interrupted |= Thread.interrupted()) && interruptible)
break;
}
spins = postSpins = (byte)((postSpins << 1) | 1);
} else
break;
}
return cancelAcquire(node, interrupted);
cancelAcquire(node);
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
}

// Cancellation support
Expand Down Expand Up @@ -1450,18 +1495,15 @@ private void unlinkCowaiter(ReaderNode node, ReaderNode leader) {
* to recheck status.
*
* @param node the waiter (may be null if not yet enqueued)
* @param interrupted if already interrupted
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero
*/
private long cancelAcquire(Node node, boolean interrupted) {
private void cancelAcquire(Node node) {
if (node != null) {
node.waiter = null;
node.status = CANCELLED;
cleanQueue();
if (node instanceof ReaderNode)
signalCowaiters((ReaderNode)node);
}
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
}

/**
Expand All @@ -1470,17 +1512,33 @@ private long cancelAcquire(Node node, boolean interrupted) {
*
* @param node if non-null, the waiter
* @param leader if non-null, the node heading cowaiters list
* @param interrupted if already interrupted
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero
*/
private long cancelCowaiter(ReaderNode node, ReaderNode leader,
boolean interrupted) {
private void cancelCowaiter(ReaderNode node, ReaderNode leader) {
if (node != null) {
node.waiter = null;
node.status = CANCELLED;
unlinkCowaiter(node, leader);
}
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
}

/**
* Fallback upon encountering OutOfMemoryErrors
*/
private long spinLockOnOOME(boolean write, boolean interruptible,
boolean timed, long time) {
long startTime = (timed) ? System.nanoTime() : 0L;
for (int spins = 0;;) {
long s = (write) ? tryAcquireWrite() : tryAcquireRead();
if (s != 0L)
return s;
Thread.onSpinWait();
if ((++spins & (1 << 8)) == 0) { // occasionally check
if (interruptible && Thread.interrupted())
return INTERRUPTED;
if (timed && System.nanoTime() - startTime > time)
return 0L;
}
}
}

// Unsafe
Expand Down
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