Centralize internal reference counting logic (#8614)
Motivation AbstractReferenceCounted and AbstractReferenceCountedByteBuf contain duplicate logic for managing the volatile refcount in an optimized and consistent manner, which increased in complexity in #8583. It's possible to extract this into a common helper class now that all access is via an AtomicIntegerFieldUpdater. Modifications - Move duplicate logic into a shared ReferenceCountUpdater class - Incorporate some additional simplification for the most common single increment/decrement cases (fewer checks/operations) Result Less code duplication, better encapsulation of the "non-trivial" internal volatile refcount manipulation
This commit is contained in:
Nick Hill
Norman Maurer
parent
e63c596f24
commit
b26a61acd1
@ -63,7 +63,7 @@ abstract class AbstractPooledDerivedByteBuf extends AbstractReferenceCountedByte
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try {
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maxCapacity(maxCapacity);
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setIndex0(readerIndex, writerIndex); // It is assumed the bounds checking is done by the caller.
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setRefCnt(1);
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resetRefCnt();
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@SuppressWarnings("unchecked")
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final U castThis = (U) this;
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@ -16,106 +16,73 @@
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package io.netty.buffer;
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import io.netty.util.IllegalReferenceCountException;
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import io.netty.util.internal.PlatformDependent;
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import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
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import static io.netty.util.internal.ObjectUtil.checkPositive;
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import io.netty.util.internal.ReferenceCountUpdater;
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/**
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* Abstract base class for {@link ByteBuf} implementations that count references.
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*/
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public abstract class AbstractReferenceCountedByteBuf extends AbstractByteBuf {
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private static final long REFCNT_FIELD_OFFSET;
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private static final AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> refCntUpdater =
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private static final long REFCNT_FIELD_OFFSET =
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ReferenceCountUpdater.getUnsafeOffset(AbstractReferenceCountedByteBuf.class, "refCnt");
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private static final AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> AIF_UPDATER =
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AtomicIntegerFieldUpdater.newUpdater(AbstractReferenceCountedByteBuf.class, "refCnt");
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// even => "real" refcount is (refCnt >>> 1); odd => "real" refcount is 0
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@SuppressWarnings("unused")
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private volatile int refCnt = 2;
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static {
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long refCntFieldOffset = -1;
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try {
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if (PlatformDependent.hasUnsafe()) {
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refCntFieldOffset = PlatformDependent.objectFieldOffset(
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AbstractReferenceCountedByteBuf.class.getDeclaredField("refCnt"));
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}
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} catch (Throwable ignore) {
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refCntFieldOffset = -1;
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private static final ReferenceCountUpdater<AbstractReferenceCountedByteBuf> updater =
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new ReferenceCountUpdater<AbstractReferenceCountedByteBuf>() {
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@Override
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protected AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> updater() {
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return AIF_UPDATER;
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}
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@Override
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protected long unsafeOffset() {
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return REFCNT_FIELD_OFFSET;
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}
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};
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REFCNT_FIELD_OFFSET = refCntFieldOffset;
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}
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private static int realRefCnt(int rawCnt) {
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return (rawCnt & 1) != 0 ? 0 : rawCnt >>> 1;
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}
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// Value might not equal "real" reference count, all access should be via the updater
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@SuppressWarnings("unused")
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private volatile int refCnt = updater.initialValue();
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protected AbstractReferenceCountedByteBuf(int maxCapacity) {
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super(maxCapacity);
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}
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private int nonVolatileRawCnt() {
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// TODO: Once we compile against later versions of Java we can replace the Unsafe usage here by varhandles.
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return REFCNT_FIELD_OFFSET != -1 ? PlatformDependent.getInt(this, REFCNT_FIELD_OFFSET)
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: refCntUpdater.get(this);
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}
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@Override
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boolean isAccessible() {
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// Try to do non-volatile read for performance as the ensureAccessible() is racy anyway and only provide
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// a best-effort guard.
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// This is copied explicitly from the nonVolatileRawCnt() method above to reduce call stack depth,
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// to avoid hitting the default limit for inlining (9)
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final int rawCnt = REFCNT_FIELD_OFFSET != -1 ? PlatformDependent.getInt(this, REFCNT_FIELD_OFFSET)
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: refCntUpdater.get(this);
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// The "real" ref count is > 0 if the rawCnt is even.
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// (x & y) appears to be surprisingly expensive relative to (x == y). Thus the expression below provides
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// a fast path for most common cases where the ref count is 1, 2, 3 or 4.
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return rawCnt == 2 || rawCnt == 4 || rawCnt == 6 || rawCnt == 8 || (rawCnt & 1) == 0;
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return updater.isLiveNonVolatile(this);
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}
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@Override
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public int refCnt() {
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return realRefCnt(refCntUpdater.get(this));
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return updater.refCnt(this);
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}
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/**
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* An unsafe operation intended for use by a subclass that sets the reference count of the buffer directly
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*/
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protected final void setRefCnt(int newRefCnt) {
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refCntUpdater.set(this, newRefCnt << 1); // overflow OK here
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protected final void setRefCnt(int refCnt) {
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updater.setRefCnt(this, refCnt);
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}
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/**
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* An unsafe operation intended for use by a subclass that resets the reference count of the buffer to 1
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*/
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protected final void resetRefCnt() {
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updater.resetRefCnt(this);
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}
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@Override
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public ByteBuf retain() {
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return retain0(1);
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return updater.retain(this);
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}
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@Override
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public ByteBuf retain(int increment) {
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return retain0(checkPositive(increment, "increment"));
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}
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private ByteBuf retain0(final int increment) {
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// all changes to the raw count are 2x the "real" change
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int adjustedIncrement = increment << 1; // overflow OK here
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int oldRef = refCntUpdater.getAndAdd(this, adjustedIncrement);
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if ((oldRef & 1) != 0) {
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throw new IllegalReferenceCountException(0, increment);
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}
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// don't pass 0!
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if ((oldRef <= 0 && oldRef + adjustedIncrement >= 0)
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|| (oldRef >= 0 && oldRef + adjustedIncrement < oldRef)) {
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// overflow case
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refCntUpdater.getAndAdd(this, -adjustedIncrement);
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throw new IllegalReferenceCountException(realRefCnt(oldRef), increment);
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}
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return this;
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return updater.retain(this, increment);
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}
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@Override
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@ -130,64 +97,19 @@ public abstract class AbstractReferenceCountedByteBuf extends AbstractByteBuf {
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@Override
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public boolean release() {
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return release0(1);
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return handleRelease(updater.release(this));
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}
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@Override
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public boolean release(int decrement) {
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return release0(checkPositive(decrement, "decrement"));
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return handleRelease(updater.release(this, decrement));
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}
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private boolean release0(int decrement) {
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int rawCnt = nonVolatileRawCnt(), realCnt = toLiveRealCnt(rawCnt, decrement);
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if (decrement == realCnt) {
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if (refCntUpdater.compareAndSet(this, rawCnt, 1)) {
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deallocate();
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return true;
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}
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return retryRelease0(decrement);
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private boolean handleRelease(boolean result) {
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if (result) {
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deallocate();
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}
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return releaseNonFinal0(decrement, rawCnt, realCnt);
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}
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private boolean releaseNonFinal0(int decrement, int rawCnt, int realCnt) {
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if (decrement < realCnt
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// all changes to the raw count are 2x the "real" change
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&& refCntUpdater.compareAndSet(this, rawCnt, rawCnt - (decrement << 1))) {
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return false;
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}
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return retryRelease0(decrement);
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}
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private boolean retryRelease0(int decrement) {
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for (;;) {
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int rawCnt = refCntUpdater.get(this), realCnt = toLiveRealCnt(rawCnt, decrement);
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if (decrement == realCnt) {
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if (refCntUpdater.compareAndSet(this, rawCnt, 1)) {
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deallocate();
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return true;
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}
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} else if (decrement < realCnt) {
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// all changes to the raw count are 2x the "real" change
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if (refCntUpdater.compareAndSet(this, rawCnt, rawCnt - (decrement << 1))) {
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return false;
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}
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} else {
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throw new IllegalReferenceCountException(realCnt, -decrement);
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}
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Thread.yield(); // this benefits throughput under high contention
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}
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}
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/**
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* Like {@link #realRefCnt(int)} but throws if refCnt == 0
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*/
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private static int toLiveRealCnt(int rawCnt, int decrement) {
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if ((rawCnt & 1) == 0) {
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return rawCnt >>> 1;
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}
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// odd rawCnt => already deallocated
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throw new IllegalReferenceCountException(0, -decrement);
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return result;
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}
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/**
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@ -1139,7 +1139,7 @@ public final class ByteBufUtil {
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static ThreadLocalUnsafeDirectByteBuf newInstance() {
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ThreadLocalUnsafeDirectByteBuf buf = RECYCLER.get();
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buf.setRefCnt(1);
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buf.resetRefCnt();
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return buf;
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}
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@ -1172,7 +1172,7 @@ public final class ByteBufUtil {
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static ThreadLocalDirectByteBuf newInstance() {
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ThreadLocalDirectByteBuf buf = RECYCLER.get();
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buf.setRefCnt(1);
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buf.resetRefCnt();
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return buf;
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}
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@ -72,7 +72,7 @@ abstract class PooledByteBuf<T> extends AbstractReferenceCountedByteBuf {
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*/
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final void reuse(int maxCapacity) {
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maxCapacity(maxCapacity);
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setRefCnt(1);
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resetRefCnt();
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setIndex0(0, 0);
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discardMarks();
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}
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@ -15,85 +15,55 @@
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*/
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package io.netty.util;
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import static io.netty.util.internal.ObjectUtil.checkPositive;
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import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
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import io.netty.util.internal.PlatformDependent;
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import io.netty.util.internal.ReferenceCountUpdater;
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/**
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* Abstract base class for classes wants to implement {@link ReferenceCounted}.
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*/
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public abstract class AbstractReferenceCounted implements ReferenceCounted {
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private static final long REFCNT_FIELD_OFFSET;
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private static final AtomicIntegerFieldUpdater<AbstractReferenceCounted> refCntUpdater =
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private static final long REFCNT_FIELD_OFFSET =
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ReferenceCountUpdater.getUnsafeOffset(AbstractReferenceCounted.class, "refCnt");
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private static final AtomicIntegerFieldUpdater<AbstractReferenceCounted> AIF_UPDATER =
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AtomicIntegerFieldUpdater.newUpdater(AbstractReferenceCounted.class, "refCnt");
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// even => "real" refcount is (refCnt >>> 1); odd => "real" refcount is 0
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@SuppressWarnings("unused")
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private volatile int refCnt = 2;
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static {
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long refCntFieldOffset = -1;
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try {
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if (PlatformDependent.hasUnsafe()) {
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refCntFieldOffset = PlatformDependent.objectFieldOffset(
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AbstractReferenceCounted.class.getDeclaredField("refCnt"));
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}
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} catch (Throwable ignore) {
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refCntFieldOffset = -1;
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private static final ReferenceCountUpdater<AbstractReferenceCounted> updater =
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new ReferenceCountUpdater<AbstractReferenceCounted>() {
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@Override
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protected AtomicIntegerFieldUpdater<AbstractReferenceCounted> updater() {
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return AIF_UPDATER;
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}
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@Override
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protected long unsafeOffset() {
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return REFCNT_FIELD_OFFSET;
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}
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};
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REFCNT_FIELD_OFFSET = refCntFieldOffset;
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}
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private static int realRefCnt(int rawCnt) {
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return (rawCnt & 1) != 0 ? 0 : rawCnt >>> 1;
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}
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private int nonVolatileRawCnt() {
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// TODO: Once we compile against later versions of Java we can replace the Unsafe usage here by varhandles.
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return REFCNT_FIELD_OFFSET != -1 ? PlatformDependent.getInt(this, REFCNT_FIELD_OFFSET)
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: refCntUpdater.get(this);
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}
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// Value might not equal "real" reference count, all access should be via the updater
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@SuppressWarnings("unused")
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private volatile int refCnt = updater.initialValue();
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@Override
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public int refCnt() {
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return realRefCnt(refCntUpdater.get(this));
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return updater.refCnt(this);
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}
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/**
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* An unsafe operation intended for use by a subclass that sets the reference count of the buffer directly
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*/
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protected final void setRefCnt(int newRefCnt) {
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refCntUpdater.set(this, newRefCnt << 1); // overflow OK here
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protected final void setRefCnt(int refCnt) {
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updater.setRefCnt(this, refCnt);
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}
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@Override
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public ReferenceCounted retain() {
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return retain0(1);
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return updater.retain(this);
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}
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@Override
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public ReferenceCounted retain(int increment) {
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return retain0(checkPositive(increment, "increment"));
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}
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private ReferenceCounted retain0(final int increment) {
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// all changes to the raw count are 2x the "real" change
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int adjustedIncrement = increment << 1; // overflow OK here
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int oldRef = refCntUpdater.getAndAdd(this, adjustedIncrement);
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if ((oldRef & 1) != 0) {
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throw new IllegalReferenceCountException(0, increment);
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}
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// don't pass 0!
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if ((oldRef <= 0 && oldRef + adjustedIncrement >= 0)
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|| (oldRef >= 0 && oldRef + adjustedIncrement < oldRef)) {
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// overflow case
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refCntUpdater.getAndAdd(this, -adjustedIncrement);
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throw new IllegalReferenceCountException(realRefCnt(oldRef), increment);
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}
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return this;
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return updater.retain(this, increment);
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}
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@Override
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@ -103,64 +73,19 @@ public abstract class AbstractReferenceCounted implements ReferenceCounted {
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@Override
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public boolean release() {
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return release0(1);
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return handleRelease(updater.release(this));
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}
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@Override
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public boolean release(int decrement) {
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return release0(checkPositive(decrement, "decrement"));
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return handleRelease(updater.release(this, decrement));
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}
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private boolean release0(int decrement) {
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int rawCnt = nonVolatileRawCnt(), realCnt = toLiveRealCnt(rawCnt, decrement);
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if (decrement == realCnt) {
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if (refCntUpdater.compareAndSet(this, rawCnt, 1)) {
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deallocate();
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return true;
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}
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return retryRelease0(decrement);
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private boolean handleRelease(boolean result) {
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if (result) {
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deallocate();
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}
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return releaseNonFinal0(decrement, rawCnt, realCnt);
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}
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private boolean releaseNonFinal0(int decrement, int rawCnt, int realCnt) {
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if (decrement < realCnt
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// all changes to the raw count are 2x the "real" change
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&& refCntUpdater.compareAndSet(this, rawCnt, rawCnt - (decrement << 1))) {
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return false;
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}
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return retryRelease0(decrement);
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}
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private boolean retryRelease0(int decrement) {
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for (;;) {
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int rawCnt = refCntUpdater.get(this), realCnt = toLiveRealCnt(rawCnt, decrement);
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if (decrement == realCnt) {
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if (refCntUpdater.compareAndSet(this, rawCnt, 1)) {
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deallocate();
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return true;
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}
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} else if (decrement < realCnt) {
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// all changes to the raw count are 2x the "real" change
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if (refCntUpdater.compareAndSet(this, rawCnt, rawCnt - (decrement << 1))) {
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return false;
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}
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} else {
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throw new IllegalReferenceCountException(realCnt, -decrement);
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}
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Thread.yield(); // this benefits throughput under high contention
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}
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}
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/**
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* Like {@link #realRefCnt(int)} but throws if refCnt == 0
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*/
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private static int toLiveRealCnt(int rawCnt, int decrement) {
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if ((rawCnt & 1) == 0) {
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return rawCnt >>> 1;
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}
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// odd rawCnt => already deallocated
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throw new IllegalReferenceCountException(0, -decrement);
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return result;
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}
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/**
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@ -0,0 +1,179 @@
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/*
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* Copyright 2019 The Netty Project
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*
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* The Netty Project licenses this file to you under the Apache License,
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* version 2.0 (the "License"); you may not use this file except in compliance
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* with the License. You may obtain a copy of the License at:
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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* License for the specific language governing permissions and limitations
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* under the License.
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*/
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package io.netty.util.internal;
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import static io.netty.util.internal.ObjectUtil.checkPositive;
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import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
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import io.netty.util.IllegalReferenceCountException;
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import io.netty.util.ReferenceCounted;
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/**
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* Common logic for {@link ReferenceCounted} implementations
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*/
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public abstract class ReferenceCountUpdater<T extends ReferenceCounted> {
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/*
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* Implementation notes:
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*
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* For the updated int field:
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* Even => "real" refcount is (refCnt >>> 1)
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* Odd => "real" refcount is 0
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*
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* (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<? extends ReferenceCounted> clz, String fieldName) {
|
||||
try {
|
||||
if (PlatformDependent.hasUnsafe()) {
|
||||
return PlatformDependent.objectFieldOffset(clz.getDeclaredField(fieldName));
|
||||
}
|
||||
} catch (Throwable ignore) {
|
||||
// fall-back
|
||||
}
|
||||
return -1;
|
||||
}
|
||||
|
||||
protected abstract AtomicIntegerFieldUpdater<T> 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
|
||||
}
|
||||
}
|
||||
}
|
||||
Loading…
Reference in New Issue
Block a user