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/*
* Copyright (c) 2009 Stanford University, unless otherwise specified.
* All rights reserved.
*
* This software was developed by the Pervasive Parallelism Laboratory of
* Stanford University, California, USA.
*
* Permission to use, copy, modify, and distribute this software in source
* or binary form for any purpose with or without fee is hereby granted,
* provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* 3. Neither the name of Stanford University nor the names of its
* contributors may be used to endorse or promote products derived
* from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
package edu.stanford.ppl.concurrent;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
/** Manages copy-on-write behavior for a concurrent tree structure. It is
* assumed that the managed structure allows concurrent mutation, but that no
* mutating operations may be active when a copy-on-write snapshot of tree is
* taken. Because it is difficult to update the size of data structure in a
* highly concurrent fashion, the CopyOnWriteManager also manages
* a running total that represents the size of the contained tree structure.
*
* Users should implement the {@link #freezeAndClone(Object)} and
* {@link #cloneFrozen(Object)} methods.
*/
abstract public class CopyOnWriteManager implements Cloneable {
/** This is basically a stripped-down CountDownLatch. Implementing our own
* reduces the object count by one, and it gives us access to the
* uninterruptable acquireShared.
*/
private class Latch extends AbstractQueuedSynchronizer {
Latch(final boolean triggered) {
setState(triggered ? 0 : 1);
}
public int tryAcquireShared(final int acquires) {
// 1 = success, and followers may also succeed
// -1 = failure
return getState() == 0 ? 1 : -1;
}
public boolean tryReleaseShared(final int releases) {
// Before, state is either 0 or 1. After, state is always 0.
return compareAndSetState(1, 0);
}
}
private static final int MUTATE = 1;
private static final int MUTATE_AFTER_FREEZE = 2;
private static final int BULK_READ = 3;
private static final int BULK_READ_AFTER_FREEZE = 4;
private class COWEpoch extends EpochNode {
/** Tripped after this COWEpoch is installed as active. */
private final Latch _activated;
/** True iff this is a mutating epoch. */
final boolean mutationAllowed;
/** The value used by this epoch. */
E value;
/** The computed size of value, as of the beginning of
* this epoch.
*/
int initialSize;
/** A frozen E equal to value, if not dirty. */
private volatile E _frozenValue;
/** True if any mutations have been performed on value. */
volatile boolean dirty;
/** The epoch that will follow this one, created on demand. */
final AtomicReference successorRef = new AtomicReference(null);
/** A ticket on the successor, released when this epoch is closed. */
Epoch.Ticket successorTicket;
/** True if the successor should freeze and clone this epoch's value. */
boolean freezeRequested;
private COWEpoch(final boolean mutationAllowed) {
this._activated = new Latch(false);
this.mutationAllowed = mutationAllowed;
}
public COWEpoch(final E value, final E frozenValue, final int initialSize) {
this._activated = new Latch(true); // pre-triggered
this.mutationAllowed = true;
this.value = value;
this.initialSize = initialSize;
this._frozenValue = frozenValue;
this.dirty = frozenValue == null;
}
EpochNode attemptInitialArrive() {
return super.attemptArrive();
}
@Override
public EpochNode attemptArrive() {
final EpochNode ticket = super.attemptArrive();
if (ticket != null && !dirty) {
dirty = true;
_frozenValue = null;
}
return ticket;
}
private void setFrozenValue(final E v) {
if (!dirty) {
_frozenValue = v;
if (dirty) {
_frozenValue = null;
}
}
}
E getFrozenValue() {
final E v = _frozenValue;
return dirty ? null : v;
}
protected void onClosed(final int dataSum) {
assert(dataSum == 0 || dirty);
final COWEpoch succ = successorRef.get();
if (freezeRequested) {
succ.value = freezeAndClone(value);
succ.setFrozenValue(value);
}
else {
succ.value = value;
if (dirty) {
succ.dirty = true;
}
else {
succ.setFrozenValue(_frozenValue);
}
}
succ.initialSize = initialSize + dataSum;
_active = succ;
successorTicket.leave(0);
succ._activated.releaseShared(1);
}
public void awaitActivated() {
_activated.acquireShared(1);
}
public COWEpoch getOrCreateSuccessor(final boolean preferredMutation) {
final COWEpoch existing = successorRef.get();
if (existing != null) {
return existing;
}
final COWEpoch repl = new COWEpoch(preferredMutation);
if (attemptInstallSuccessor(repl)) {
return repl;
}
return successorRef.get();
}
public boolean attemptInstallSuccessor(final COWEpoch succ) {
final Epoch.Ticket t = succ.attemptInitialArrive();
if (successorRef.compareAndSet(null, succ)) {
successorTicket = t;
beginClose();
return true;
}
else {
return false;
}
}
}
private volatile COWEpoch _active;
/** Creates a new {@link CopyOnWriteManager} holding
* initialValue, with an assumed size of
* initialSize.
*/
public CopyOnWriteManager(final E initialValue, final int initialSize) {
_active = new COWEpoch(initialValue, null, initialSize);
}
/** The implementing method must mark value as shared, and
* return a new object to use in its place. Hopefully, the majority of
* the work of the clone can be deferred by copy-on-write.
*/
abstract protected E freezeAndClone(final E value);
/** Returns a clone of a frozen E. */
abstract protected E cloneFrozen(E frozenValue);
public CopyOnWriteManager clone() {
final CopyOnWriteManager copy;
try {
copy = (CopyOnWriteManager) super.clone();
}
catch (final CloneNotSupportedException xx) {
throw new Error("unexpected", xx);
}
COWEpoch a = _active;
E f = a.getFrozenValue();
while (f == null) {
a.freezeRequested = true;
final COWEpoch succ = a.getOrCreateSuccessor(a.mutationAllowed);
succ.awaitActivated();
if (a.value != succ.value) {
f = a.value;
}
a = succ;
}
copy.createNewEpoch(f, a);
return copy;
}
private void createNewEpoch(E f, COWEpoch a)
{
_active = new COWEpoch(cloneFrozen(f), f, a.initialSize);
}
/** Returns a reference to the tree structure suitable for a read
* operation. The returned structure may be mutated by operations that
* have the permission of this {@link CopyOnWriteManager}, but they will
* not observe changes managed by other instances.
*/
public E read() {
return _active.value;
}
/** Obtains permission to mutate the copy-on-write value held by this
* instance, perhaps blocking while a concurrent snapshot is being
* performed. {@link Epoch.Ticket#leave} must be called exactly once on
* the object returned from this method, after the mutation has been
* completed. The change in size reflected by the mutation should be
* passed as the parameter to leave.
*/
public Epoch.Ticket beginMutation() {
return begin(true);
}
public Epoch.Ticket beginQuiescent() {
return begin(false);
}
private Epoch.Ticket begin(final boolean mutation) {
final COWEpoch active = _active;
if (active.mutationAllowed == mutation) {
final Epoch.Ticket ticket = active.attemptArrive();
if (ticket != null) {
return ticket;
}
}
return begin(mutation, active);
}
private Epoch.Ticket begin(final boolean mutation, COWEpoch epoch) {
while (true) {
COWEpoch succ = epoch.successorRef.get();
if (succ == null) {
final COWEpoch newEpoch = new COWEpoch(mutation);
final Epoch.Ticket newTicket = newEpoch.attemptArrive();
if (epoch.attemptInstallSuccessor(newEpoch)) {
// can't use the ticket until the new epoch is activated
newEpoch.awaitActivated();
return newTicket;
}
// if our CAS failed, somebody else succeeded
succ = epoch.successorRef.get();
}
// is the successor created by someone else suitable?
if (succ.mutationAllowed == mutation) {
final Epoch.Ticket ticket = succ.attemptArrive();
if (ticket != null) {
succ.awaitActivated();
return ticket;
}
}
epoch = succ;
}
}
/** Returns a reference to the tree structure suitable for a mutating
* operation. This method may only be called under the protection of a
* ticket returned from {@link #beginMutation}.
*/
public E mutable() {
return _active.value;
}
/** Returns a reference to a snapshot of this instance's tree structure
* that may be read, but not written. This is accomplished by suspending
* mutation, replacing the mutable root of this manager with the result of
* freezeAndClone(root, false), and then returning a
* reference to the old root. Successive calls to this method may return
* the same instance.
*/
public E frozen() {
COWEpoch a = _active;
E f = a.getFrozenValue();
while (f == null) {
a.freezeRequested = true;
final COWEpoch succ = a.getOrCreateSuccessor(a.mutationAllowed);
succ.awaitActivated();
if (a.value != succ.value) {
f = a.value;
}
a = succ;
}
return f;
}
/** Returns a reference to a snapshot of this instance's tree structure,
* if one is available without requiring any additional copying, otherwise
* returns null. May be used in combination with {@link #beginQuiescent}
* to perform quiescent reads with minimal cost.
*/
public E availableFrozen() {
return _active.getFrozenValue();
}
/** Returns true if the computed {@link #size} is zero. */
public boolean isEmpty() {
// for a different internal implementation (such as a C-SNZI) we might
// be able to do better than this
return size() == 0;
}
/** Returns the sum of the initialSize parameter passed to the
* constructor, and the size deltas passed to {@link Epoch.Ticket#leave}
* for all of the mutation tickets. The result returned is linearizable
* with mutations, which requires mutation to be quiesced. No tree freeze
* is required, however.
*/
public int size() {
final COWEpoch a = _active;
final Integer delta = a.attemptDataSum();
if (delta != null) {
return a.initialSize + delta;
}
// wait for an existing successor, or force one if not already in progress
final COWEpoch succ = a.getOrCreateSuccessor(a.mutationAllowed);
succ.awaitActivated();
return succ.initialSize;
}
}