// Unsafe mechanics
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, String field, Class<?> klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
}
/** Node class for TransferStacks. */
static final class SNode {
volatile SNode next; // next node in stack
volatile SNode match; // the node matched to this
volatile Thread waiter; // to control park/unpark
Object item; // data; or null for REQUESTs
int mode;
// Note: item and mode fields don't need to be volatile
// since they are always written before, and read after,
// other volatile/atomic operations.
SNode(Object item) {
this.item = item;
}
boolean casNext(SNode cmp, SNode val) {
return cmp == next && UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
/**
* Tries to match node s to this node, if so, waking up thread. Fulfillers call tryMatch to
* identify their waiters. Waiters block until they have been matched.
*
* @param s the node to match
* @return true if successfully matched to s
*/
boolean tryMatch(SNode s) {
if (match == null && UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) {
Thread w = waiter;
if (w != null) { // waiters need at most one unpark
waiter = null;
LockSupport.unpark(w);
}
return true;
}
return match == s;
}
/** Tries to cancel a wait by matching node to itself. */
void tryCancel() {
UNSAFE.compareAndSwapObject(this, matchOffset, null, this);
}
boolean isCancelled() {
return match == this;
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long nextOffset = objectFieldOffset(UNSAFE, "next", SNode.class);
private static final long matchOffset = objectFieldOffset(UNSAFE, "match", SNode.class);
}
/** Node class for TransferQueue. */
static final class QNode {
volatile QNode next; // next node in queue
volatile Object item; // CAS'ed to or from null
volatile Thread waiter; // to control park/unpark
final boolean isData;
QNode(Object item, boolean isData) {
this.item = item;
this.isData = isData;
}
boolean casNext(QNode cmp, QNode val) {
return next == cmp && UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
boolean casItem(Object cmp, Object val) {
return item == cmp && UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
}
/** Tries to cancel by CAS'ing ref to this as item. */
void tryCancel(Object cmp) {
UNSAFE.compareAndSwapObject(this, itemOffset, cmp, this);
}
boolean isCancelled() {
return item == this;
}
/**
* Returns true if this node is known to be off the queue because its next pointer has been
* forgotten due to an advanceHead operation.
*/
boolean isOffList() {
return next == this;
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long nextOffset = objectFieldOffset(UNSAFE, "next", QNode.class);
private static final long itemOffset = objectFieldOffset(UNSAFE, "item", QNode.class);
}
/** Dual Queue */
static final class TransferQueue extends Transferer {
/*
* This extends Scherer-Scott dual queue algorithm, differing,
* among other ways, by using modes within nodes rather than
* marked pointers. The algorithm is a little simpler than
* that for stacks because fulfillers do not need explicit
* nodes, and matching is done by CAS'ing QNode.item field
* from non-null to null (for put) or vice versa (for take).
*/
/** Node class for TransferQueue. */
static final class QNode {
volatile QNode next; // next node in queue
volatile Object item; // CAS'ed to or from null
volatile Thread waiter; // to control park/unpark
final boolean isData;
QNode(Object item, boolean isData) {
this.item = item;
this.isData = isData;
}
boolean casNext(QNode cmp, QNode val) {
return next == cmp && UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
boolean casItem(Object cmp, Object val) {
return item == cmp && UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
}
/** Tries to cancel by CAS'ing ref to this as item. */
void tryCancel(Object cmp) {
UNSAFE.compareAndSwapObject(this, itemOffset, cmp, this);
}
boolean isCancelled() {
return item == this;
}
/**
* Returns true if this node is known to be off the queue because its next pointer has been
* forgotten due to an advanceHead operation.
*/
boolean isOffList() {
return next == this;
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long nextOffset = objectFieldOffset(UNSAFE, "next", QNode.class);
private static final long itemOffset = objectFieldOffset(UNSAFE, "item", QNode.class);
}
/** Head of queue */
transient volatile QNode head;
/** Tail of queue */
transient volatile QNode tail;
/**
* Reference to a cancelled node that might not yet have been unlinked from queue because it was
* the last inserted node when it cancelled.
*/
transient volatile QNode cleanMe;
TransferQueue() {
QNode h = new QNode(null, false); // initialize to dummy node.
head = h;
tail = h;
}
/**
* Tries to cas nh as new head; if successful, unlink old head's next node to avoid garbage
* retention.
*/
void advanceHead(QNode h, QNode nh) {
if (h == head && UNSAFE.compareAndSwapObject(this, headOffset, h, nh))
h.next = h; // forget old next
}
/** Tries to cas nt as new tail. */
void advanceTail(QNode t, QNode nt) {
if (tail == t) UNSAFE.compareAndSwapObject(this, tailOffset, t, nt);
}
/** Tries to CAS cleanMe slot. */
boolean casCleanMe(QNode cmp, QNode val) {
return cleanMe == cmp && UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
}
/** Puts or takes an item. */
Object transfer(Object e, boolean timed, long nanos) {
/* Basic algorithm is to loop trying to take either of
* two actions:
*
* 1. If queue apparently empty or holding same-mode nodes,
* try to add node to queue of waiters, wait to be
* fulfilled (or cancelled) and return matching item.
*
* 2. If queue apparently contains waiting items, and this
* call is of complementary mode, try to fulfill by CAS'ing
* item field of waiting node and dequeuing it, and then
* returning matching item.
*
* In each case, along the way, check for and try to help
* advance head and tail on behalf of other stalled/slow
* threads.
*
* The loop starts off with a null check guarding against
* seeing uninitialized head or tail values. This never
* happens in current SynchronousQueue, but could if
* callers held non-volatile/final ref to the
* transferer. The check is here anyway because it places
* null checks at top of loop, which is usually faster
* than having them implicitly interspersed.
*/
QNode s = null; // constructed/reused as needed
boolean isData = (e != null);
for (; ; ) {
QNode t = tail;
QNode h = head;
if (t == null || h == null) // saw uninitialized value
continue; // spin
if (h == t || t.isData == isData) { // empty or same-mode
QNode tn = t.next;
if (t != tail) // inconsistent read
continue;
if (tn != null) { // lagging tail
advanceTail(t, tn);
continue;
}
if (timed && nanos <= 0) // can't wait
return null;
if (s == null) s = new QNode(e, isData);
if (!t.casNext(null, s)) // failed to link in
continue;
advanceTail(t, s); // swing tail and wait
Object x = awaitFulfill(s, e, timed, nanos);
if (x == s) { // wait was cancelled
clean(t, s);
return null;
}
if (!s.isOffList()) { // not already unlinked
advanceHead(t, s); // unlink if head
if (x != null) // and forget fields
s.item = s;
s.waiter = null;
}
return (x != null) ? x : e;
} else { // complementary-mode
QNode m = h.next; // node to fulfill
if (t != tail || m == null || h != head) continue; // inconsistent read
Object x = m.item;
if (isData == (x != null)
|| // m already fulfilled
x == m
|| // m cancelled
!m.casItem(x, e)) { // lost CAS
advanceHead(h, m); // dequeue and retry
continue;
}
advanceHead(h, m); // successfully fulfilled
LockSupport.unpark(m.waiter);
return (x != null) ? x : e;
}
}
}
/**
* Spins/blocks until node s is fulfilled.
*
* @param s the waiting node
* @param e the comparison value for checking match
* @param timed true if timed wait
* @param nanos timeout value
* @return matched item, or s if cancelled
*/
Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
/* Same idea as TransferStack.awaitFulfill */
long lastTime = timed ? System.nanoTime() : 0;
Thread w = Thread.currentThread();
int spins = ((head.next == s) ? (timed ? maxTimedSpins : maxUntimedSpins) : 0);
for (; ; ) {
if (w.isInterrupted()) s.tryCancel(e);
Object x = s.item;
if (x != e) return x;
if (timed) {
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
if (nanos <= 0) {
s.tryCancel(e);
continue;
}
}
if (spins > 0) --spins;
else if (s.waiter == null) s.waiter = w;
else if (!timed) LockSupport.park(this);
else if (nanos > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanos);
}
}
/** Gets rid of cancelled node s with original predecessor pred. */
void clean(QNode pred, QNode s) {
s.waiter = null; // forget thread
/*
* At any given time, exactly one node on list cannot be
* deleted -- the last inserted node. To accommodate this,
* if we cannot delete s, we save its predecessor as
* "cleanMe", deleting the previously saved version
* first. At least one of node s or the node previously
* saved can always be deleted, so this always terminates.
*/
while (pred.next == s) { // Return early if already unlinked
QNode h = head;
QNode hn = h.next; // Absorb cancelled first node as head
if (hn != null && hn.isCancelled()) {
advanceHead(h, hn);
continue;
}
QNode t = tail; // Ensure consistent read for tail
if (t == h) return;
QNode tn = t.next;
if (t != tail) continue;
if (tn != null) {
advanceTail(t, tn);
continue;
}
if (s != t) { // If not tail, try to unsplice
QNode sn = s.next;
if (sn == s || pred.casNext(s, sn)) return;
}
QNode dp = cleanMe;
if (dp != null) { // Try unlinking previous cancelled node
QNode d = dp.next;
QNode dn;
if (d == null
|| // d is gone or
d == dp
|| // d is off list or
!d.isCancelled()
|| // d not cancelled or
(d != t
&& // d not tail and
(dn = d.next) != null
&& // has successor
dn != d
&& // that is on list
dp.casNext(d, dn))) // d unspliced
casCleanMe(dp, null);
if (dp == pred) return; // s is already saved node
} else if (casCleanMe(null, pred)) return; // Postpone cleaning s
}
}
// unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long headOffset = objectFieldOffset(UNSAFE, "head", TransferQueue.class);
private static final long tailOffset = objectFieldOffset(UNSAFE, "tail", TransferQueue.class);
private static final long cleanMeOffset =
objectFieldOffset(UNSAFE, "cleanMe", TransferQueue.class);
}
/** Dual stack */
static final class TransferStack extends Transferer {
/*
* This extends Scherer-Scott dual stack algorithm, differing,
* among other ways, by using "covering" nodes rather than
* bit-marked pointers: Fulfilling operations push on marker
* nodes (with FULFILLING bit set in mode) to reserve a spot
* to match a waiting node.
*/
/* Modes for SNodes, ORed together in node fields */
/** Node represents an unfulfilled consumer */
static final int REQUEST = 0;
/** Node represents an unfulfilled producer */
static final int DATA = 1;
/** Node is fulfilling another unfulfilled DATA or REQUEST */
static final int FULFILLING = 2;
/** Return true if m has fulfilling bit set */
static boolean isFulfilling(int m) {
return (m & FULFILLING) != 0;
}
/** Node class for TransferStacks. */
static final class SNode {
volatile SNode next; // next node in stack
volatile SNode match; // the node matched to this
volatile Thread waiter; // to control park/unpark
Object item; // data; or null for REQUESTs
int mode;
// Note: item and mode fields don't need to be volatile
// since they are always written before, and read after,
// other volatile/atomic operations.
SNode(Object item) {
this.item = item;
}
boolean casNext(SNode cmp, SNode val) {
return cmp == next && UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
/**
* Tries to match node s to this node, if so, waking up thread. Fulfillers call tryMatch to
* identify their waiters. Waiters block until they have been matched.
*
* @param s the node to match
* @return true if successfully matched to s
*/
boolean tryMatch(SNode s) {
if (match == null && UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) {
Thread w = waiter;
if (w != null) { // waiters need at most one unpark
waiter = null;
LockSupport.unpark(w);
}
return true;
}
return match == s;
}
/** Tries to cancel a wait by matching node to itself. */
void tryCancel() {
UNSAFE.compareAndSwapObject(this, matchOffset, null, this);
}
boolean isCancelled() {
return match == this;
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long nextOffset = objectFieldOffset(UNSAFE, "next", SNode.class);
private static final long matchOffset = objectFieldOffset(UNSAFE, "match", SNode.class);
}
/** The head (top) of the stack */
volatile SNode head;
boolean casHead(SNode h, SNode nh) {
return h == head && UNSAFE.compareAndSwapObject(this, headOffset, h, nh);
}
/**
* Creates or resets fields of a node. Called only from transfer where the node to push on stack
* is lazily created and reused when possible to help reduce intervals between reads and CASes
* of head and to avoid surges of garbage when CASes to push nodes fail due to contention.
*/
static SNode snode(SNode s, Object e, SNode next, int mode) {
if (s == null) s = new SNode(e);
s.mode = mode;
s.next = next;
return s;
}
/** Puts or takes an item. */
Object transfer(Object e, boolean timed, long nanos) {
/*
* Basic algorithm is to loop trying one of three actions:
*
* 1. If apparently empty or already containing nodes of same
* mode, try to push node on stack and wait for a match,
* returning it, or null if cancelled.
*
* 2. If apparently containing node of complementary mode,
* try to push a fulfilling node on to stack, match
* with corresponding waiting node, pop both from
* stack, and return matched item. The matching or
* unlinking might not actually be necessary because of
* other threads performing action 3:
*
* 3. If top of stack already holds another fulfilling node,
* help it out by doing its match and/or pop
* operations, and then continue. The code for helping
* is essentially the same as for fulfilling, except
* that it doesn't return the item.
*/
SNode s = null; // constructed/reused as needed
int mode = (e == null) ? REQUEST : DATA;
for (; ; ) {
SNode h = head;
if (h == null || h.mode == mode) { // empty or same-mode
if (timed && nanos <= 0) { // can't wait
if (h != null && h.isCancelled()) casHead(h, h.next); // pop cancelled node
else return null;
} else if (casHead(h, s = snode(s, e, h, mode))) {
SNode m = awaitFulfill(s, timed, nanos);
if (m == s) { // wait was cancelled
clean(s);
return null;
}
if ((h = head) != null && h.next == s) casHead(h, s.next); // help s's fulfiller
return (mode == REQUEST) ? m.item : s.item;
}
} else if (!isFulfilling(h.mode)) { // try to fulfill
if (h.isCancelled()) // already cancelled
casHead(h, h.next); // pop and retry
else if (casHead(h, s = snode(s, e, h, FULFILLING | mode))) {
for (; ; ) { // loop until matched or waiters disappear
SNode m = s.next; // m is s's match
if (m == null) { // all waiters are gone
casHead(s, null); // pop fulfill node
s = null; // use new node next time
break; // restart main loop
}
SNode mn = m.next;
if (m.tryMatch(s)) {
casHead(s, mn); // pop both s and m
return (mode == REQUEST) ? m.item : s.item;
} else // lost match
s.casNext(m, mn); // help unlink
}
}
} else { // help a fulfiller
SNode m = h.next; // m is h's match
if (m == null) // waiter is gone
casHead(h, null); // pop fulfilling node
else {
SNode mn = m.next;
if (m.tryMatch(h)) // help match
casHead(h, mn); // pop both h and m
else // lost match
h.casNext(m, mn); // help unlink
}
}
}
}
/**
* Spins/blocks until node s is matched by a fulfill operation.
*
* @param s the waiting node
* @param timed true if timed wait
* @param nanos timeout value
* @return matched node, or s if cancelled
*/
SNode awaitFulfill(SNode s, boolean timed, long nanos) {
/*
* When a node/thread is about to block, it sets its waiter
* field and then rechecks state at least one more time
* before actually parking, thus covering race vs
* fulfiller noticing that waiter is non-null so should be
* woken.
*
* When invoked by nodes that appear at the point of call
* to be at the head of the stack, calls to park are
* preceded by spins to avoid blocking when producers and
* consumers are arriving very close in time. This can
* happen enough to bother only on multiprocessors.
*
* The order of checks for returning out of main loop
* reflects fact that interrupts have precedence over
* normal returns, which have precedence over
* timeouts. (So, on timeout, one last check for match is
* done before giving up.) Except that calls from untimed
* SynchronousQueue.{poll/offer} don't check interrupts
* and don't wait at all, so are trapped in transfer
* method rather than calling awaitFulfill.
*/
long lastTime = timed ? System.nanoTime() : 0;
Thread w = Thread.currentThread();
SNode h = head;
int spins = (shouldSpin(s) ? (timed ? maxTimedSpins : maxUntimedSpins) : 0);
for (; ; ) {
if (w.isInterrupted()) s.tryCancel();
SNode m = s.match;
if (m != null) return m;
if (timed) {
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
if (nanos <= 0) {
s.tryCancel();
continue;
}
}
if (spins > 0) spins = shouldSpin(s) ? (spins - 1) : 0;
else if (s.waiter == null) s.waiter = w; // establish waiter so can park next iter
else if (!timed) LockSupport.park(this);
else if (nanos > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanos);
}
}
/** Returns true if node s is at head or there is an active fulfiller. */
boolean shouldSpin(SNode s) {
SNode h = head;
return (h == s || h == null || isFulfilling(h.mode));
}
/** Unlinks s from the stack. */
void clean(SNode s) {
s.item = null; // forget item
s.waiter = null; // forget thread
/*
* At worst we may need to traverse entire stack to unlink
* s. If there are multiple concurrent calls to clean, we
* might not see s if another thread has already removed
* it. But we can stop when we see any node known to
* follow s. We use s.next unless it too is cancelled, in
* which case we try the node one past. We don't check any
* further because we don't want to doubly traverse just to
* find sentinel.
*/
SNode past = s.next;
if (past != null && past.isCancelled()) past = past.next;
// Absorb cancelled nodes at head
SNode p;
while ((p = head) != null && p != past && p.isCancelled()) casHead(p, p.next);
// Unsplice embedded nodes
while (p != null && p != past) {
SNode n = p.next;
if (n != null && n.isCancelled()) p.casNext(n, n.next);
else p = n;
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long headOffset = objectFieldOffset(UNSAFE, "head", TransferStack.class);
}