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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.atomic.AtomicBoolean;
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 = 2048;
// 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, ReentrantLock> locks =
new ConcurrentHashMap<>(1024, .75f, NUM_STRIPES);
public RefCountedMultisetKeyedLocker() {
}
private class RefCountedAutoUnlocker implements AutoUnlocker {
private final K key;
private final ReentrantLock lock;
private final AtomicBoolean closeCalled = new AtomicBoolean(false);
private RefCountedAutoUnlocker(K key, ReentrantLock lock) {
this.key = key;
this.lock = lock;
}
@Override
public void close() {
if (closeCalled.getAndSet(true)) {
String msg = String.format("For key %s, 'close' can be called at most once per "
+ "AutoUnlocker instance", key);
throw new IllegalUnlockException(msg);
}
if (!lock.isHeldByCurrentThread()) {
String msg = String.format("For key %s, the calling thread to 'close' must be the one "
+ "that acquired the AutoUnlocker", key);
throw new IllegalUnlockException(msg);
}
try {
Lock waitersLock = waitersLocks.get(key);
try {
waitersLock.lock();
// Note that ConcurrentHashMultiset automatically removes removes entries for keys whose
// count is 0.
if (waiters.remove(key, 1) == 1) {
// No other thread is waiting to access this key, nor does the current thread have
// another AutoUnlocker instance, so we garbage collect the lock.
Preconditions.checkState(locks.remove(key, lock), key);
}
} finally {
waitersLock.unlock();
}
} finally {
lock.unlock();
}
}
}
@Override
public AutoUnlocker lock(K key) {
ReentrantLock newLock = new ReentrantLock();
// 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. If we already have access to 'key', this simply bumps up
// the ref count.
waiters.add(key);
} finally {
waitersLock.unlock();
}
ReentrantLock lock;
lock = locks.putIfAbsent(key, newLock);
if (lock != null) {
Preconditions.checkState(lock != newLock);
newLock.unlock();
// Either another thread won the race to get access to 'key', or we already have exclusive
// access to 'key'. Either way, we lock; in the former case we wait for our turn and in the
// latter case the lock's implicit counter is incremented.
lock.lock();
return new RefCountedAutoUnlocker(key, lock);
}
// We won the race, so the current lock for 'key' is the one we locked and inserted.
return new RefCountedAutoUnlocker(key, newLock);
}
}
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