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/*
* Copyright 2019 The Netty Project
*
* The Netty Project licenses this file to you under the Apache License,
* version 2.0 (the "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at:
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations
* under the License.
*/
package io.netty.util.internal;
import static io.netty.util.internal.ObjectUtil.checkPositive;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
import io.netty.util.IllegalReferenceCountException;
import io.netty.util.ReferenceCounted;
/**
* Common logic for {@link ReferenceCounted} implementations
*/
public abstract class ReferenceCountUpdater {
/*
* Implementation notes:
*
* For the updated int field:
* Even => "real" refcount is (refCnt >>> 1)
* Odd => "real" refcount is 0
*
* (x & y) appears to be surprisingly expensive relative to (x == y). Thus this class uses
* a fast-path in some places for most common low values when checking for live (even) refcounts,
* for example: if (rawCnt == 2 || rawCnt == 4 || (rawCnt & 1) == 0) { ...
*/
protected ReferenceCountUpdater() { }
public static long getUnsafeOffset(Class clz, String fieldName) {
try {
if (PlatformDependent.hasUnsafe()) {
return PlatformDependent.objectFieldOffset(clz.getDeclaredField(fieldName));
}
} catch (Throwable ignore) {
// fall-back
}
return -1;
}
protected abstract AtomicIntegerFieldUpdater updater();
protected abstract long unsafeOffset();
public final int initialValue() {
return 2;
}
private static int realRefCnt(int rawCnt) {
return rawCnt != 2 && rawCnt != 4 && (rawCnt & 1) != 0 ? 0 : rawCnt >>> 1;
}
/**
* Like {@link #realRefCnt(int)} but throws if refCnt == 0
*/
private static int toLiveRealRefCnt(int rawCnt, int decrement) {
if (rawCnt == 2 || rawCnt == 4 || (rawCnt & 1) == 0) {
return rawCnt >>> 1;
}
// odd rawCnt => already deallocated
throw new IllegalReferenceCountException(0, -decrement);
}
private int nonVolatileRawCnt(T instance) {
// TODO: Once we compile against later versions of Java we can replace the Unsafe usage here by varhandles.
final long offset = unsafeOffset();
return offset != -1 ? PlatformDependent.getInt(instance, offset) : updater().get(instance);
}
public final int refCnt(T instance) {
return realRefCnt(updater().get(instance));
}
public final boolean isLiveNonVolatile(T instance) {
final long offset = unsafeOffset();
final int rawCnt = offset != -1 ? PlatformDependent.getInt(instance, offset) : updater().get(instance);
// The "real" ref count is > 0 if the rawCnt is even.
return rawCnt == 2 || rawCnt == 4 || rawCnt == 6 || rawCnt == 8 || (rawCnt & 1) == 0;
}
/**
* An unsafe operation that sets the reference count directly
*/
public final void setRefCnt(T instance, int refCnt) {
updater().set(instance, refCnt > 0 ? refCnt << 1 : 1); // overflow OK here
}
/**
* Resets the reference count to 1
*/
public final void resetRefCnt(T instance) {
updater().set(instance, initialValue());
}
public final T retain(T instance) {
return retain0(instance, 1, 2);
}
public final T retain(T instance, int increment) {
// all changes to the raw count are 2x the "real" change - overflow is OK
int rawIncrement = checkPositive(increment, "increment") << 1;
return retain0(instance, increment, rawIncrement);
}
// rawIncrement == increment << 1
private T retain0(T instance, final int increment, final int rawIncrement) {
int oldRef = updater().getAndAdd(instance, rawIncrement);
if (oldRef != 2 && oldRef != 4 && (oldRef & 1) != 0) {
throw new IllegalReferenceCountException(0, increment);
}
// don't pass 0!
if ((oldRef <= 0 && oldRef + rawIncrement >= 0)
|| (oldRef >= 0 && oldRef + rawIncrement < oldRef)) {
// overflow case
updater().getAndAdd(instance, -rawIncrement);
throw new IllegalReferenceCountException(realRefCnt(oldRef), increment);
}
return instance;
}
public final boolean release(T instance) {
int rawCnt = nonVolatileRawCnt(instance);
return rawCnt == 2 ? tryFinalRelease0(instance, 2) || retryRelease0(instance, 1)
: nonFinalRelease0(instance, 1, rawCnt, toLiveRealRefCnt(rawCnt, 1));
}
public final boolean release(T instance, int decrement) {
int rawCnt = nonVolatileRawCnt(instance);
int realCnt = toLiveRealRefCnt(rawCnt, checkPositive(decrement, "decrement"));
return decrement == realCnt ? tryFinalRelease0(instance, rawCnt) || retryRelease0(instance, decrement)
: nonFinalRelease0(instance, decrement, rawCnt, realCnt);
}
private boolean tryFinalRelease0(T instance, int expectRawCnt) {
return updater().compareAndSet(instance, expectRawCnt, 1); // any odd number will work
}
private boolean nonFinalRelease0(T instance, int decrement, int rawCnt, int realCnt) {
if (decrement < realCnt
// all changes to the raw count are 2x the "real" change - overflow is OK
&& updater().compareAndSet(instance, rawCnt, rawCnt - (decrement << 1))) {
return false;
}
return retryRelease0(instance, decrement);
}
private boolean retryRelease0(T instance, int decrement) {
for (;;) {
int rawCnt = updater().get(instance), realCnt = toLiveRealRefCnt(rawCnt, decrement);
if (decrement == realCnt) {
if (tryFinalRelease0(instance, rawCnt)) {
return true;
}
} else if (decrement < realCnt) {
// all changes to the raw count are 2x the "real" change
if (updater().compareAndSet(instance, rawCnt, rawCnt - (decrement << 1))) {
return false;
}
} else {
throw new IllegalReferenceCountException(realCnt, -decrement);
}
Thread.yield(); // this benefits throughput under high contention
}
}
}