org.neo4j.kernel.impl.locking.community.RWLock Maven / Gradle / Ivy
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/*
* Copyright (c) 2002-2015 "Neo Technology,"
* Network Engine for Objects in Lund AB [http://neotechnology.com]
*
* This file is part of Neo4j.
*
* Neo4j is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see
* When a transaction has write lock no other tx is allowed to acquire read or
* write lock on that resource but the tx holding the write lock. If one tx has
* acquired write lock and another tx needs a lock on the same resource that tx
* must wait. When the lock is released the other tx is notified and wakes up so
* it can acquire the lock.
*
* Waiting for locks may lead to a deadlock. Consider the following scenario. Tx
* T1 acquires write lock on resource R1. T2 acquires write lock on R2. Now T1
* tries to acquire read lock on R2 but has to wait since R2 is locked by T2. If
* T2 now tries to acquire a lock on R1 it also has to wait because R1 is locked
* by T1. T2 cannot wait on R1 because that would lead to a deadlock where T1
* and T2 waits forever.
*
* Avoiding deadlocks can be done by keeping a resource allocation graph. This
* class works together with the {@link RagManager} to make sure no deadlocks
* occur.
*
* Waiting transactions are put into a queue and when some tx releases the lock
* the queue is checked for waiting txs. This implementation tries to avoid lock
* starvation and increase performance since only waiting txs that can acquire
* the lock are notified.
*/
class RWLock
{
private final Object resource; // the resource this RWLock locks
private final LinkedList waitingThreadList = new LinkedList<>();
private final ArrayMap txLockElementMap = new ArrayMap<>( (byte)5, false, true );
private final RagManager ragManager;
// access to these is guarded by synchronized blocks
private int totalReadCount;
private int totalWriteCount;
private int marked; // synch helper in LockManager
RWLock( Object resource, RagManager ragManager )
{
this.resource = resource;
this.ragManager = ragManager;
}
// keeps track of a transactions read and write lock count on this RWLock
private static class TxLockElement
{
private final Object tx;
// access to these is guarded by synchronized blocks
private int readCount;
private int writeCount;
// represent number of active request that where current TxLockElement participate in
// as soon as hasNoRequests return true - txLockElement can be cleaned up
private int requests = 0;
// flag indicate that current TxLockElement is terminated because owning client closed
private boolean terminated = false;
TxLockElement( Object tx )
{
this.tx = tx;
}
void incrementRequests()
{
requests++;
}
void decrementRequests()
{
requests--;
}
boolean hasNoRequests()
{
return requests == 0;
}
boolean isFree()
{
return readCount == 0 && writeCount == 0;
}
public boolean isTerminated()
{
return terminated;
}
public void setTerminated( boolean terminated )
{
this.terminated = terminated;
}
}
// keeps track of what type of lock a thread is waiting for
private static class LockRequest
{
private final TxLockElement element;
private final LockType lockType;
private final Thread waitingThread;
private final long since = System.currentTimeMillis();
LockRequest( TxLockElement element, LockType lockType, Thread thread )
{
this.element = element;
this.lockType = lockType;
this.waitingThread = thread;
}
}
public Object resource()
{
return resource;
}
synchronized void mark()
{
this.marked++;
}
synchronized boolean isMarked()
{
return marked > 0;
}
/**
* Tries to acquire read lock for a given transaction. If
* this.writeCount is greater than the currents tx's write
* count the transaction has to wait and the {@link RagManager#checkWaitOn}
* method is invoked for deadlock detection.
*
* If the lock can be acquired the lock count is updated on this
* and the transaction lock element (tle).
* Waiting for a lock can also be terminated. In that case waiting thread will be interrupted and corresponding
* {@link org.neo4j.kernel.impl.locking.community.RWLock.TxLockElement} will be marked as terminated.
* In that case lock will not be acquired and false will be return as result of acquisition
*
* @throws DeadlockDetectedException
* if a deadlock is detected
* @return true is lock was acquired, false otherwise
*/
synchronized boolean acquireReadLock( Object tx ) throws DeadlockDetectedException
{
TxLockElement tle = getOrCreateLockElement( tx );
LockRequest lockRequest = null;
// used to track do we need to add lock request to a waiting queue or we still have it there
boolean addLockRequest = true;
try
{
tle.incrementRequests();
Thread currentThread = currentThread();
while ( !tle.isTerminated() && (totalWriteCount > tle.writeCount) )
{
ragManager.checkWaitOn( this, tx );
if ( addLockRequest )
{
lockRequest = new LockRequest( tle, READ, currentThread );
waitingThreadList.addFirst( lockRequest );
}
try
{
wait();
addLockRequest = false;
}
catch ( InterruptedException e )
{
interrupted();
addLockRequest = true;
}
ragManager.stopWaitOn( this, tx );
}
if ( !tle.isTerminated() )
{
registerReadLockAcquired( tx, tle );
return true;
} else {
// in case if lock element was interrupted and it was never register before
// we need to clean it from lock element map
// if it was register before it will be cleaned up during standard lock release call
if ( tle.requests == 1 && tle.isFree() )
{
txLockElementMap.remove( tx );
}
return false;
}
}
finally
{
cleanupWaitingListRequests( lockRequest, tle, addLockRequest );
// for cases when spurious wake up was the reason why we waked up, but also there
// was an interruption as described at 17.2 just clearing interruption flag
interrupted();
// if deadlocked, remove marking so lock is removed when empty
tle.decrementRequests();
marked--;
}
}
synchronized boolean tryAcquireReadLock( Object tx )
{
TxLockElement tle = getOrCreateLockElement( tx );
try
{
if ( tle.isTerminated() || (totalWriteCount > tle.writeCount) )
{
return false;
}
registerReadLockAcquired( tx, tle );
return true;
}
finally
{
// if deadlocked, remove marking so lock is removed when empty
marked--;
}
}
/**
* Releases the read lock held by the provided transaction. If it is null then
* an attempt to acquire the current transaction will be made. This is to
* make safe calling the method from the context of an
* afterCompletion() hook where the tx is locally stored and
* not necessarily available through the tm. If there are waiting
* transactions in the queue they will be interrupted if they can acquire
* the lock.
*/
synchronized void releaseReadLock( Object tx ) throws LockNotFoundException
{
TxLockElement tle = getLockElement( tx );
if ( tle.readCount == 0 )
{
throw new LockNotFoundException( "" + tx + " don't have readLock" );
}
totalReadCount--;
tle.readCount--;
if ( tle.isFree() )
{
ragManager.lockReleased( this, tx );
if ( tle.hasNoRequests() )
{
txLockElementMap.remove( tx );
}
}
if ( !waitingThreadList.isEmpty() )
{
LockRequest lockRequest = waitingThreadList.getLast();
if ( lockRequest.lockType == LockType.WRITE )
{
// this one is tricky...
// if readCount > 0 lockRequest either have to find a waiting read lock
// in the queue or a waiting write lock that has all read
// locks, if none of these are found it means that there
// is a (are) thread(s) that will release read lock(s) in the
// near future...
if ( totalReadCount == lockRequest.element.readCount )
{
// found a write lock with all read locks
waitingThreadList.removeLast();
lockRequest.waitingThread.interrupt();
}
else
{
ListIterator listItr = waitingThreadList.listIterator(
waitingThreadList.lastIndexOf( lockRequest ) );
// hm am I doing the first all over again?
// think I am if cursor is at lastIndex + 0.5 oh well...
while ( listItr.hasPrevious() )
{
lockRequest = listItr.previous();
if ( lockRequest.lockType == LockType.WRITE && totalReadCount == lockRequest.element.readCount )
{
// found a write lock with all read locks
listItr.remove();
lockRequest.waitingThread.interrupt();
break;
}
else if ( lockRequest.lockType == LockType.READ )
{
// found a read lock, let it do the job...
listItr.remove();
lockRequest.waitingThread.interrupt();
}
}
}
}
else
{
// some thread may have the write lock and released a read lock
// if writeCount is down to zero lockRequest can interrupt the waiting
// read lock
if ( totalWriteCount == 0 )
{
waitingThreadList.removeLast();
lockRequest.waitingThread.interrupt();
}
}
}
}
/**
* Tries to acquire write lock for a given transaction. If
* this.writeCount is greater than the currents tx's write
* count or the read count is greater than the currents tx's read count the
* transaction has to wait and the {@link RagManager#checkWaitOn} method is
* invoked for deadlock detection.
*
* If the lock can be acquires the lock count is updated on this
* and the transaction lock element (tle).
* Waiting for a lock can also be terminated. In that case waiting thread will be interrupted and corresponding
* {@link org.neo4j.kernel.impl.locking.community.RWLock.TxLockElement} will be marked as terminated.
* In that case lock will not be acquired and false will be return as result of acquisition
*
* @throws DeadlockDetectedException
* if a deadlock is detected
* @return true is lock was acquired, false otherwise
*/
synchronized boolean acquireWriteLock( Object tx ) throws DeadlockDetectedException
{
TxLockElement tle = getOrCreateLockElement( tx );
LockRequest lockRequest = null;
// used to track do we need to add lock request to a waiting queue or we still have it there
boolean addLockRequest = true;
try
{
tle.incrementRequests();
Thread currentThread = currentThread();
while ( !tle.isTerminated() && (totalWriteCount > tle.writeCount || totalReadCount > tle.readCount) )
{
ragManager.checkWaitOn( this, tx );
if ( addLockRequest )
{
lockRequest = new LockRequest( tle, WRITE, currentThread );
waitingThreadList.addFirst( lockRequest );
}
try
{
wait();
addLockRequest = false;
}
catch ( InterruptedException e )
{
interrupted();
addLockRequest = true;
}
ragManager.stopWaitOn( this, tx );
}
if ( !tle.isTerminated() )
{
registerWriteLockAcquired( tx, tle );
return true;
} else {
// in case if lock element was interrupted and it was never register before
// we need to clean it from lock element map
// if it was register before it will be cleaned up during standard lock release call
if ( tle.requests == 1 && tle.isFree() )
{
txLockElementMap.remove( tx );
}
return false;
}
}
finally
{
cleanupWaitingListRequests( lockRequest, tle, addLockRequest );
// for cases when spurious wake up was the reason why we waked up, but also there
// was an interruption as described at 17.2 just clearing interruption flag
interrupted();
// if deadlocked, remove marking so lock is removed when empty
tle.decrementRequests();
marked--;
}
}
// in case of spurious wake up, deadlock during spurious wake up, termination
// when we already have request in a queue we need to clean it up
private void cleanupWaitingListRequests( LockRequest lockRequest, TxLockElement lockElement,
boolean addLockRequest )
{
if ( lockRequest != null && (lockElement.isTerminated() || !addLockRequest) )
{
waitingThreadList.remove( lockRequest );
}
}
synchronized boolean tryAcquireWriteLock( Object tx )
{
TxLockElement tle = getOrCreateLockElement( tx );
try
{
if ( tle.isTerminated() || (totalWriteCount > tle.writeCount) || (totalReadCount > tle.readCount) )
{
return false;
}
registerWriteLockAcquired( tx, tle );
return true;
}
finally
{
// if deadlocked, remove marking so lock is removed when empty
marked--;
}
}
/**
* Releases the write lock held by the provided tx. If it is null then an
* attempt to acquire the current transaction from the transaction manager
* will be made. This is to make safe calling this method as an
* afterCompletion() hook where the transaction context is not
* necessarily available. If write count is zero and there are waiting
* transactions in the queue they will be interrupted if they can acquire
* the lock.
*/
synchronized void releaseWriteLock( Object tx ) throws LockNotFoundException
{
TxLockElement tle = getLockElement( tx );
if ( tle.writeCount == 0 )
{
throw new LockNotFoundException( "" + tx + " don't have writeLock" );
}
totalWriteCount--;
tle.writeCount--;
if ( tle.isFree() )
{
ragManager.lockReleased( this, tx );
if ( tle.hasNoRequests() )
{
txLockElementMap.remove( tx );
}
}
// the threads in the waitingList cannot be currentThread
// so we only have to wake other elements if writeCount is down to zero
// (that is: If writeCount > 0 a waiting thread in the queue cannot be
// the thread that holds the write locks because then it would never
// have been put into wait mode)
if ( totalWriteCount == 0 && waitingThreadList.size() > 0 )
{
// wake elements in queue until a write lock is found or queue is
// empty
do
{
LockRequest lockRequest = waitingThreadList.removeLast();
lockRequest.waitingThread.interrupt();
if ( lockRequest.lockType == LockType.WRITE )
{
break;
}
}
while ( !waitingThreadList.isEmpty() );
}
}
synchronized int getWriteCount()
{
return totalWriteCount;
}
synchronized int getReadCount()
{
return totalReadCount;
}
synchronized int getWaitingThreadsCount()
{
return waitingThreadList.size();
}
public synchronized boolean logTo( Logger logger )
{
logger.log( "Total lock count: readCount=" + totalReadCount
+ " writeCount=" + totalWriteCount + " for " + resource );
logger.log( "Waiting list:" );
Iterator wElements = waitingThreadList.iterator();
while ( wElements.hasNext() )
{
LockRequest lockRequest = wElements.next();
logger.log( "[" + lockRequest.waitingThread + "("
+ lockRequest.element.readCount + "r," + lockRequest.element.writeCount + "w),"
+ lockRequest.lockType + "]" );
if ( wElements.hasNext() )
{
logger.log( "," );
}
else
{
logger.log( "" );
}
}
logger.log( "Locking transactions:" );
Iterator lElements = txLockElementMap.values().iterator();
while ( lElements.hasNext() )
{
TxLockElement tle = lElements.next();
logger.log( "" + tle.tx + "(" + tle.readCount + "r,"
+ tle.writeCount + "w)" );
}
return true;
}
public synchronized String describe()
{
StringBuilder sb = new StringBuilder( this.toString() );
sb.append( " Total lock count: readCount=" + totalReadCount
+ " writeCount=" + totalWriteCount + " for " + resource + "\n" )
.append( "Waiting list:" + "\n" );
Iterator wElements = waitingThreadList.iterator();
while ( wElements.hasNext() )
{
LockRequest lockRequest = wElements.next();
sb.append( "[" + lockRequest.waitingThread + "("
+ lockRequest.element.readCount + "r," + lockRequest.element.writeCount + "w),"
+ lockRequest.lockType + "]\n" );
if ( wElements.hasNext() )
{
sb.append( "," );
}
}
sb.append( "Locking transactions:\n" );
Iterator lElements = txLockElementMap.values().iterator();
while ( lElements.hasNext() )
{
TxLockElement tle = lElements.next();
sb.append( "" + tle.tx + "(" + tle.readCount + "r,"
+ tle.writeCount + "w)\n" );
}
return sb.toString();
}
public synchronized long maxWaitTime()
{
long max = 0l;
for ( LockRequest thread : waitingThreadList )
{
if ( thread.since < max )
{
max = thread.since;
}
}
return System.currentTimeMillis() - max;
}
// for specified transaction object mark all lock elements as terminated
// and interrupt all waiters
synchronized void terminateLockRequestsForLockTransaction(Object lockTransaction) {
TxLockElement lockElement = txLockElementMap.get( lockTransaction );
if ( lockElement != null && !lockElement.isTerminated())
{
lockElement.setTerminated( true );
for ( LockRequest lockRequest : waitingThreadList )
{
if ( lockRequest.element.tx.equals( lockTransaction ) )
{
lockRequest.waitingThread.interrupt();
}
}
}
}
@Override
public String toString()
{
return "RWLock[" + resource + ", hash="+hashCode()+"]";
}
private void registerReadLockAcquired( Object tx, TxLockElement tle )
{
registerLockAcquired( tx, tle );
totalReadCount++;
tle.readCount++;
}
private void registerWriteLockAcquired( Object tx, TxLockElement tle )
{
registerLockAcquired( tx, tle );
totalWriteCount++;
tle.writeCount++;
}
private void registerLockAcquired( Object tx, TxLockElement tle )
{
if ( tle.isFree() )
{
ragManager.lockAcquired( this, tx );
}
}
private TxLockElement getLockElement( Object tx )
{
TxLockElement tle = txLockElementMap.get( tx );
if ( tle == null )
{
throw new LockNotFoundException( "No transaction lock element found for " + tx );
}
return tle;
}
private void assertTransaction( Object tx )
{
if ( tx == null )
{
throw new IllegalArgumentException();
}
}
private TxLockElement getOrCreateLockElement( Object tx )
{
assertTransaction( tx );
TxLockElement tle = txLockElementMap.get( tx );
if ( tle == null )
{
txLockElementMap.put( tx, tle = new TxLockElement( tx ) );
}
return tle;
}
synchronized Object getTxLockElementCount()
{
return txLockElementMap.size();
}
}