/*
* Copyright 2016 higherfrequencytrading.com
*
* Licensed 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 net.openhft.lang.locks;
public final class VanillaReadWriteWithWaitsLockingStrategy extends AbstractReadWriteLockingStrategy
implements ReadWriteWithWaitsLockingStrategy {
static final int RW_LOCK_LIMIT = 30;
static final long RW_READ_LOCKED = 1L;
static final long RW_WRITE_WAITING = 1L << RW_LOCK_LIMIT;
static final long RW_WRITE_LOCKED = 1L << 2 * RW_LOCK_LIMIT;
static final int RW_LOCK_MASK = (1 << RW_LOCK_LIMIT) - 1;
private static final ReadWriteWithWaitsLockingStrategy INSTANCE =
new VanillaReadWriteWithWaitsLockingStrategy();
private VanillaReadWriteWithWaitsLockingStrategy() {
}
public static ReadWriteWithWaitsLockingStrategy instance() {
return INSTANCE;
}
static int rwReadLocked(long lock) {
return (int) (lock & RW_LOCK_MASK);
}
static int rwWriteWaiting(long lock) {
return (int) ((lock >>> RW_LOCK_LIMIT) & RW_LOCK_MASK);
}
static int rwWriteLocked(long lock) {
return (int) (lock >>> (2 * RW_LOCK_LIMIT));
}
static <T> long read(NativeAtomicAccess<T> access, T t, long offset) {
return access.getLongVolatile(t, offset);
}
static <T> boolean cas(NativeAtomicAccess<T> access, T t, long offset, long expected, long x) {
return access.compareAndSwapLong(t, offset, expected, x);
}
@Override
public <T> boolean tryReadLock(NativeAtomicAccess<T> access, T t, long offset) {
long lock = read(access, t, offset);
int writersWaiting = rwWriteWaiting(lock);
int writersLocked = rwWriteLocked(lock);
// readers wait for waiting writers
if (writersLocked <= 0 && writersWaiting <= 0) {
// increment readers locked.
int readersLocked = rwReadLocked(lock);
if (readersLocked >= RW_LOCK_MASK)
throw new IllegalMonitorStateException("readersLocked has reached a limit of " +
readersLocked);
if (cas(access, t, offset, lock, lock + RW_READ_LOCKED))
return true;
}
return false;
}
@Override
public <T> boolean tryWriteLock(NativeAtomicAccess<T> access, T t, long offset) {
long lock = read(access, t, offset);
int readersLocked = rwReadLocked(lock);
int writersLocked = rwWriteLocked(lock);
// writers don't wait for waiting readers.
if (readersLocked <= 0 && writersLocked <= 0) {
if (cas(access, t, offset, lock, lock + RW_WRITE_LOCKED))
return true;
}
return false;
}
@Override
public <T> boolean tryUpgradeReadToWriteLock(NativeAtomicAccess<T> access, T t, long offset) {
throw new UnsupportedOperationException("not implemented yet");
}
@Override
public <T> void readUnlock(NativeAtomicAccess<T> access, T t, long offset) {
for (; ; ) {
long lock = read(access, t, offset);
int readersLocked = rwReadLocked(lock);
if (readersLocked <= 0)
throw new IllegalMonitorStateException("readerLock underflow");
if (cas(access, t, offset, lock, lock - RW_READ_LOCKED))
return;
}
}
@Override
public <T> void writeUnlock(NativeAtomicAccess<T> access, T t, long offset) {
for (; ; ) {
long lock = read(access, t, offset);
int writersLocked = rwWriteLocked(lock);
if (writersLocked != 1)
throw new IllegalMonitorStateException("writersLock underflow " + writersLocked);
if (cas(access, t, offset, lock, lock - RW_WRITE_LOCKED))
return;
}
}
@Override
public <T> void downgradeWriteToReadLock(NativeAtomicAccess<T> access, T t, long offset) {
throw new UnsupportedOperationException("not implemented yet");
}
@Override
public boolean isWriteLocked(long state) {
return rwWriteLocked(state) > 0;
}
@Override
public int readLockCount(long state) {
return rwReadLocked(state);
}
@Override
public <T> void reset(NativeAtomicAccess<T> access, T t, long offset) {
access.putOrderedLong(t, offset, 0L);
}
@Override
public <T> void resetKeepingWaits(NativeAtomicAccess<T> access, T t, long offset) {
while (true) {
long lock = read(access, t, offset);
long onlyWaits = lock & ((long) RW_LOCK_MASK) << RW_LOCK_LIMIT;
if (cas(access, t, offset, lock, onlyWaits))
return;
}
}
@Override
public <T> void registerWait(NativeAtomicAccess<T> access, T t, long offset) {
for (; ; ) {
long lock = read(access, t, offset);
int writersWaiting = rwWriteWaiting(lock);
if (writersWaiting >= RW_LOCK_MASK)
throw new IllegalMonitorStateException("writersWaiting has reached a limit of " +
writersWaiting);
if (cas(access, t, offset, lock, lock + RW_WRITE_WAITING))
break;
}
}
@Override
public <T> void deregisterWait(NativeAtomicAccess<T> access, T t, long offset) {
for (; ; ) {
long lock = read(access, t, offset);
int writersWaiting = rwWriteWaiting(lock);
if (writersWaiting <= 0)
throw new IllegalMonitorStateException("writersWaiting has underflowed");
if (cas(access, t, offset, lock, lock - RW_WRITE_WAITING))
break;
}
}
@Override
public <T> boolean tryWriteLockAndDeregisterWait(
NativeAtomicAccess<T> access, T t, long offset) {
long lock = read(access, t, offset);
int readersLocked = rwReadLocked(lock);
int writersWaiting = rwWriteWaiting(lock);
int writersLocked = rwWriteLocked(lock);
if (readersLocked <= 0 && writersLocked <= 0) {
// increment readers locked.
if (writersWaiting <= 0)
throw new IllegalMonitorStateException("writersWaiting has underflowed");
// add to the readLock count and decrease the readWaiting count.
if (cas(access, t, offset, lock, lock + RW_WRITE_LOCKED - RW_WRITE_WAITING))
return true;
}
return false;
}
@Override
public <T> boolean tryUpgradeReadToWriteLockAndDeregisterWait(
NativeAtomicAccess<T> access, T t, long offset) {
throw new UnsupportedOperationException("not implemented yet");
}
@Override
public long resetState() {
return 0L;
}
@Override
public <T> long getState(NativeAtomicAccess<T> access, T t, long offset) {
return read(access, t, offset);
}
@Override
public int waitCount(long state) {
return rwWriteWaiting(state);
}
@Override
public boolean isLocked(long state) {
return isReadLocked(state) || isWriteLocked(state);
}
@Override
public int lockCount(long state) {
return rwReadLocked(state) + rwWriteLocked(state);
}
@Override
public String toString(long state) {
return "[read locks = " + readLockCount(state) +
", write locked = " + isWriteLocked(state) +
", waits = " + waitCount(state) + "]";
}
@Override
public int sizeInBytes() {
return 8;
}
}