/**
* If node non-null, forces cancel status and unsplices it from queue if possible and wakes up any
* cowaiters (of the node, or group, as applicable), and in any case helps release current first
* waiter if lock is free. (Calling with null arguments serves as a conditional form of release,
* which is not currently needed but may be needed under possible future cancellation policies).
* This is a variant of cancellation methods in AbstractQueuedSynchronizer (see its detailed
* explanation in AQS internal documentation).
*
* @param node if nonnull, the waiter
* @param group either node or the group node is cowaiting with
* @param interrupted if already interrupted
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero
*/
private long cancelWaiter(WNode node, WNode group, boolean interrupted) {
if (node != null && group != null) {
Thread w;
node.status = CANCELLED;
// unsplice cancelled nodes from group
for (WNode p = group, q; (q = p.cowait) != null; ) {
if (q.status == CANCELLED) {
U.compareAndSwapObject(p, WCOWAIT, q, q.cowait);
p = group; // restart
} else p = q;
}
if (group == node) {
for (WNode r = group.cowait; r != null; r = r.cowait) {
if ((w = r.thread) != null) U.unpark(w); // wake up uncancelled co-waiters
}
for (WNode pred = node.prev; pred != null; ) { // unsplice
WNode succ, pp; // find valid successor
while ((succ = node.next) == null || succ.status == CANCELLED) {
WNode q = null; // find successor the slow way
for (WNode t = wtail; t != null && t != node; t = t.prev)
if (t.status != CANCELLED) q = t; // don't link if succ cancelled
if (succ == q
|| // ensure accurate successor
U.compareAndSwapObject(node, WNEXT, succ, succ = q)) {
if (succ == null && node == wtail) U.compareAndSwapObject(this, WTAIL, node, pred);
break;
}
}
if (pred.next == node) // unsplice pred link
U.compareAndSwapObject(pred, WNEXT, node, succ);
if (succ != null && (w = succ.thread) != null) {
succ.thread = null;
U.unpark(w); // wake up succ to observe new pred
}
if (pred.status != CANCELLED || (pp = pred.prev) == null) break;
node.prev = pp; // repeat if new pred wrong/cancelled
U.compareAndSwapObject(pp, WNEXT, pred, succ);
pred = pp;
}
}
}
WNode h; // Possibly release first waiter
while ((h = whead) != null) {
long s;
WNode q; // similar to release() but check eligibility
if ((q = h.next) == null || q.status == CANCELLED) {
for (WNode t = wtail; t != null && t != h; t = t.prev) if (t.status <= 0) q = t;
}
if (h == whead) {
if (q != null
&& h.status == 0
&& ((s = state) & ABITS) != WBIT
&& // waiter is eligible
(s == 0L || q.mode == RMODE)) release(h);
break;
}
}
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
}
/**
* See above for explanation.
*
* @param interruptible true if should check interrupts and if so return INTERRUPTED
* @param deadline if nonzero, the System.nanoTime value to timeout at (and return zero)
* @return next state, or INTERRUPTED
*/
private long acquireRead(boolean interruptible, long deadline) {
WNode node = null, p;
boolean interrupted = false;
for (int spins = -1; ; ) {
WNode h;
if ((h = whead) == (p = wtail)) {
for (long m, s, ns; ; ) {
if ((m = (s = state) & ABITS) < RFULL
? U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT)
: (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
if (interrupted) Thread.currentThread().interrupt();
return ns;
} else if (m >= WBIT) {
if (spins > 0) {
if (LockSupport.nextSecondarySeed() >= 0) --spins;
} else {
if (spins == 0) {
WNode nh = whead, np = wtail;
if ((nh == h && np == p) || (h = nh) != (p = np)) break;
}
spins = SPINS;
}
}
}
}
if (p == null) { // initialize queue
WNode hd = new WNode(WMODE, null);
if (U.compareAndSwapObject(this, WHEAD, null, hd)) wtail = hd;
} else if (node == null) node = new WNode(RMODE, p);
else if (h == p || p.mode != RMODE) {
if (node.prev != p) node.prev = p;
else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
p.next = node;
break;
}
} else if (!U.compareAndSwapObject(p, WCOWAIT, node.cowait = p.cowait, node))
node.cowait = null;
else {
for (; ; ) {
WNode pp, c;
Thread w;
if ((h = whead) != null
&& (c = h.cowait) != null
&& U.compareAndSwapObject(h, WCOWAIT, c, c.cowait)
&& (w = c.thread) != null) // help release
U.unpark(w);
if (h == (pp = p.prev) || h == p || pp == null) {
long m, s, ns;
do {
if ((m = (s = state) & ABITS) < RFULL
? U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT)
: (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
if (interrupted) Thread.currentThread().interrupt();
return ns;
}
} while (m < WBIT);
}
if (whead == h && p.prev == pp) {
long time;
if (pp == null || h == p || p.status > 0) {
node = null; // throw away
break;
}
if (deadline == 0L) time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L)
return cancelWaiter(node, p, false);
Thread wt = Thread.currentThread();
U.putObject(wt, PARKBLOCKER, this);
node.thread = wt;
if ((h != pp || (state & ABITS) == WBIT) && whead == h && p.prev == pp)
U.park(false, time);
node.thread = null;
U.putObject(wt, PARKBLOCKER, null);
// if (interruptible && Thread.interrupted())
// return cancelWaiter(node, p, true);
if (Thread.interrupted()) {
if (interruptible) return cancelWaiter(node, p, true);
else interrupted = true;
}
}
}
}
}
for (int spins = -1; ; ) {
WNode h, np, pp;
int ps;
if ((h = whead) == p) {
if (spins < 0) spins = HEAD_SPINS;
else if (spins < MAX_HEAD_SPINS) spins <<= 1;
for (int k = spins; ; ) { // spin at head
long m, s, ns;
if ((m = (s = state) & ABITS) < RFULL
? U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT)
: (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
WNode c;
Thread w;
whead = node;
node.prev = null;
while ((c = node.cowait) != null) {
if (U.compareAndSwapObject(node, WCOWAIT, c, c.cowait) && (w = c.thread) != null)
U.unpark(w);
}
if (interrupted) Thread.currentThread().interrupt();
return ns;
} else if (m >= WBIT && LockSupport.nextSecondarySeed() >= 0 && --k <= 0) break;
}
} else if (h != null) {
WNode c;
Thread w;
while ((c = h.cowait) != null) {
if (U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) && (w = c.thread) != null)
U.unpark(w);
}
}
if (whead == h) {
if ((np = node.prev) != p) {
if (np != null) (p = np).next = node; // stale
} else if ((ps = p.status) == 0) U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
else if (ps == CANCELLED) {
if ((pp = p.prev) != null) {
node.prev = pp;
pp.next = node;
}
} else {
long time;
if (deadline == 0L) time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L)
return cancelWaiter(node, node, false);
Thread wt = Thread.currentThread();
U.putObject(wt, PARKBLOCKER, this);
node.thread = wt;
if (p.status < 0 && (p != h || (state & ABITS) == WBIT) && whead == h && node.prev == p)
U.park(false, time);
node.thread = null;
U.putObject(wt, PARKBLOCKER, null);
// if (interruptible && Thread.interrupted())
// return cancelWaiter(node, node, true);
if (Thread.interrupted()) {
if (interruptible) return cancelWaiter(node, p, true);
else interrupted = true;
}
}
}
}
}
/**
* See above for explanation.
*
* @param interruptible true if should check interrupts and if so return INTERRUPTED
* @param deadline if nonzero, the System.nanoTime value to timeout at (and return zero)
* @return next state, or INTERRUPTED
*/
private long acquireWrite(boolean interruptible, long deadline) {
WNode node = null, p;
boolean interrupted = false;
for (int spins = -1; ; ) { // spin while enqueuing
long m, s, ns;
if ((m = (s = state) & ABITS) == 0L) {
if (U.compareAndSwapLong(this, STATE, s, ns = s + WBIT)) {
if (interrupted) Thread.currentThread().interrupt();
return ns;
}
} else if (spins < 0) spins = (m == WBIT && wtail == whead) ? SPINS : 0;
else if (spins > 0) {
if (LockSupport.nextSecondarySeed() >= 0) --spins;
} else if ((p = wtail) == null) { // initialize queue
WNode hd = new WNode(WMODE, null);
if (U.compareAndSwapObject(this, WHEAD, null, hd)) wtail = hd;
} else if (node == null) node = new WNode(WMODE, p);
else if (node.prev != p) node.prev = p;
else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
p.next = node;
break;
}
}
for (int spins = -1; ; ) {
WNode h, np, pp;
int ps;
if ((h = whead) == p) {
if (spins < 0) spins = HEAD_SPINS;
else if (spins < MAX_HEAD_SPINS) spins <<= 1;
for (int k = spins; ; ) { // spin at head
long s, ns;
if (((s = state) & ABITS) == 0L) {
if (U.compareAndSwapLong(this, STATE, s, ns = s + WBIT)) {
whead = node;
node.prev = null;
if (interrupted) Thread.currentThread().interrupt();
return ns;
}
} else if (LockSupport.nextSecondarySeed() >= 0 && --k <= 0) break;
}
} else if (h != null) { // help release stale waiters
WNode c;
Thread w;
while ((c = h.cowait) != null) {
if (U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) && (w = c.thread) != null)
U.unpark(w);
}
}
if (whead == h) {
if ((np = node.prev) != p) {
if (np != null) (p = np).next = node; // stale
} else if ((ps = p.status) == 0) U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
else if (ps == CANCELLED) {
if ((pp = p.prev) != null) {
node.prev = pp;
pp.next = node;
}
} else {
long time; // 0 argument to park means no timeout
if (deadline == 0L) time = 0L;
else if ((time = deadline - System.nanoTime()) <= 0L)
return cancelWaiter(node, node, false);
Thread wt = Thread.currentThread();
U.putObject(wt, PARKBLOCKER, this);
node.thread = wt;
if (p.status < 0 && (p != h || (state & ABITS) != 0L) && whead == h && node.prev == p)
U.park(false, time); // emulate LockSupport.park
node.thread = null;
U.putObject(wt, PARKBLOCKER, null);
// if (interruptible && Thread.interrupted())
// return cancelWaiter(node, node, true);
if (Thread.interrupted()) {
if (interruptible) return cancelWaiter(node, node, true);
else interrupted = true;
}
}
}
}
}
