/**
* 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);
}
}
/**
* 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;
}
/**
* 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);
}
}
/** 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;
}
}
}