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// Copyright 2015 Google Inc. All rights reserved.
//
// 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 com.google.devtools.build.lib.concurrent;
import com.google.common.base.Preconditions;
import com.google.common.collect.ConcurrentHashMultiset;
import com.google.common.util.concurrent.Striped;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* An implementation of {@link KeyedLocker} that uses ref counting to efficiently only store locks
* that are live.
*/
public class RefCountedMultisetKeyedLocker<K> implements KeyedLocker<K> {
// Multiset of keys that have threads waiting on a lock or using a lock.
private final ConcurrentHashMultiset<K> waiters = ConcurrentHashMultiset.<K>create();
private static final int NUM_STRIPES = 256;
// For each key, gives the striped lock to use for atomically managing the waiters on that key
// internally.
private final Striped<Lock> waitersLocks = Striped.lazyWeakLock(NUM_STRIPES);
// Map of key to current lock, for keys that have at least one waiting or live thread.
private final ConcurrentMap<K, RefCountedLockImpl> locks = new ConcurrentHashMap<>();
private class RefCountedLockImpl extends ReentrantLock implements AutoUnlocker {
private final K key;
private RefCountedLockImpl(K key) {
this.key = key;
}
@Override
public void close() {
Preconditions.checkState(isHeldByCurrentThread(), "For key %s, 'close' can be called at most "
+ "once and the calling thread must be the one that acquired the AutoUnlocker", key);
try {
Lock waitersLock = waitersLocks.get(key);
try {
waitersLock.lock();
// Note that ConcurrentHashMultiset automatically removes removes entries for keys whose
// count is 0.
waiters.remove(key);
if (waiters.count(key) == 0) {
// No other thread is waiting to access this key, so we garbage collect the lock.
Preconditions.checkState(locks.remove(key, this), key);
}
} finally {
waitersLock.unlock();
}
} finally {
unlock();
}
}
}
@Override
public AutoUnlocker lock(K key) {
RefCountedLockImpl newLock = new RefCountedLockImpl(key);
// Pre-lock our fresh lock, in case we win the race to get access to 'key'.
newLock.lock();
Lock waitersLock = waitersLocks.get(key);
try {
waitersLock.lock();
// Add us to the set of waiters, so that in case we lose the race to access 'key', the winner
// will know that we are waiting.
waiters.add(key);
} finally {
waitersLock.unlock();
}
RefCountedLockImpl lock;
lock = locks.putIfAbsent(key, newLock);
if (lock != null) {
// Another thread won the race to get access to 'key', so we wait for our turn.
Preconditions.checkState(lock != newLock);
newLock.unlock();
lock.lock();
return lock;
}
// We won the race, so the current lock for 'key' is the one we locked and inserted.
return newLock;
}
}
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