ReentrantReadWriteLock 卡在解锁状态

Question

我有一个 class 用于获取和释放文件的锁。我使用的 customKey class 只是一个带有 id 字符串（id 是文件）的 ReentrantReadWriteLock。出于某种原因，这仅在某些情况下有效，并且在大多数情况下它会挂起所有东西的解锁 - 我的调试器跟踪它一直在那里使用然后就卡住了。

我做错了什么？如果一个线程崩溃并且没有释放它的锁，我会得到，但是这里一个线程试图调用解锁并且没有进一步。

这是锁定的方法：

override fun acquire(lockId: String?, ownerId: String?, sequence: Long): Boolean
{
    if (lockId != null)
    {
        lockedList.find { customLock -> customLock.Id == lockId }.apply {
            if (this != null) //lock already exists for this ID
            {
                println("Locking file $lockId Existing lock")
                this.writeLock().lock()
                println("Locked file $lockId")
            } else //lock does not exist
            {
                val newLock = CustomLock(lockId)
                lockedList.add(newLock)
                println("Locking file $lockId")
                newLock.writeLock().lock()
                println("Locked file $lockId")
            }
        }
        return true
    } else
    {
        throw InvalidParameterException("ERROR: lockId or ownerId is null!")
    }
}

释放方法是这样的：

override fun release(lockId: String?, ownerId: String?)
    {
        if (lockId != null)
        {
            lockedList.find { customLock -> customLock.Id == lockId }.apply {
                if (this != null)
                {
                    println("Unlocking file $lockId")
                    this.writeLock().unlock()
                    if (this.isWriteLocked)
                    {
                        throw Exception("ERROR: Unlocking failed!")
                    }
                } else
                {
                    throw Exception("ERROR: Lock not found!")
                }
            }
        }
    }

体系结构就别讲了，这个是作业写的。另外请忽略 ownerId 和序列变量。

编辑：我尝试只使用一个锁，虽然效率不高，但它确实有效，所以@gidds 可能有问题，但 ConcurrentHashMap 和 ConcurrentLinkedQueue（替换 List 更简单）都没有解决问题.

EDIT2：这是我使用 ConcurrentHashMap 的新 class。它仍然无法正常工作，任何人都可以指出我搞砸的地方吗？谢谢

class LockServer(port: Int) : LockConnector, RemoteException()
{
private val lockedList = ConcurrentHashMap<String, CustomLock>()
private var registry: Registry = LocateRegistry.createRegistry(port)

init
{
    registry.bind(ServiceNames.LockService.toString(), UnicastRemoteObject.exportObject(this, port))
}

/**
 * Method acquire() should block the multiple calls from the clients for each specific lockId string.
 * It means when one client acquires the lock "A" and the "A" is not locked by any other clients,
 * the method should record the lock and return true. If the "A" is already locked by any other client,
 * the method is blocked and continues only after the lock "A" is released.
 * (Note: Return value false is not used in this basic implementation.
 * Parameters ownerId and sequence are also not used in this basic implementation.)
 */
override fun acquire(lockId: String?, ownerId: String?, sequence: Long): Boolean
{
    if (lockId != null)
    {
        lockedList.computeIfPresent(lockId){id, value ->
            println("Locking file $id Existing lock")
            value.writeLock().lock()
            println("Locked file $id")
            return@computeIfPresent value
        }
        lockedList.computeIfAbsent(lockId){
            val newLock = CustomLock(it)
            println("Locking file $lockId")
            newLock.writeLock().lock()
            println("Locked file $lockId")
            return@computeIfAbsent newLock
        }
        return true
    } else
    {
        throw InvalidParameterException("ERROR: lockId or ownerId is null!")
    }
}

/**
 * Method release() should release the lock and unblock all waiting acquire() calls for the same lock.
 * (Note: Parameter ownerId is not used in this basic implementation.)
 */
override fun release(lockId: String?, ownerId: String?)
{
    if (lockId != null)
    {
        lockedList.computeIfPresent(lockId){ id, value ->
            println("Unlocking file $id")
            value.writeLock().unlock()
            println("Unlocked file $id")
            return@computeIfPresent value
        }
    }
}

/**
 * Method stop() unbinds the current server object from the RMI registry and unexports it.
 */
override fun stop()
{
    registry.unbind(ServiceNames.LockService.toString())
}

}

编辑3： acquire 的新实现：

lockedList.compute(lockId){id, value ->
            if (value == null)
            {
                println("Locking file $id")
                val newLock = CustomLock(id)
                newLock.writeLock().lock()
                println("Locked file $id")
                return@compute newLock
            }
            println("Locking file $id Existing lock")
            value.writeLock().lock()
            println("Locked file $id")
            return@compute value
        }

并发布：

println("Unlocking $lockId")
        lockedList[lockId]!!.writeLock().unlock()
        println("Unlocked $lockId")

还是一样的失败

Answer 1

这可能不是您的问题，但代码在添加新锁时存在竞争条件：如果两个线程尝试锁定同一个（新）文件，则它们都可以为该文件创建一个锁。两把锁都会被添加到列表中，但之后只会找到第一把。（这假定列表本身是 thread-safe；否则其中一个添加可能会失败、永远循环或使列表处于不一致状态并稍后崩溃。）

你可以通过一些同步来解决这个问题。但更好的方法可能是将锁存储在 ConcurrentHashMap (keyed on the lock ID) instead of a list, and use an atomic operation such as computeIfAbsent() 中以安全地创建新锁。（这也将提高渐近性能，因为它可以避免每次扫描列表。）

此外，作为风格问题，在锁上使用 apply() 看起来有点别扭。（它的通常用途是自定义 newly-created 对象。）我认为 let() 在那里会更惯用；您只需要将 this 更改为 it 即可。当然，也可以使用 old-fashioned 临时变量。

Answer 2

这可能不是 LockServer class 的问题，而是正在使用它的问题：

线程 1：

acquire("file1")
acquire("file2")
release("file2")
release("file1")

线程 2：

acquire("file2")
acquire("file1")
release("file1")
release("file2")

而恰好执行顺序如下：

thread1.acquire("file1")
thread2.acquire("file2")
thread1.acquire("file2") //locked by thread2, waiting
thread2.acquire("file1") //locked by thread1... BOOM, deadlock!

更新：

考虑对现有锁使用 tryLock()（可能有一些超时）而不是简单的 lock()：

    fun tryAcquire(lockId: String?, timeout: Long, unit: TimeUnit): Boolean {
        if (lockId != null) {
            var success = false
            lockedList.compute(lockId) { id, value ->
                if (value == null) {
                    println("Locking file $id")
                    val newLock = CustomLock(id)
                    newLock.writeLock().lock()
                    success = true
                    println("Locked file $id")
                    return@compute newLock
                }
                println("Locking file $id Existing lock")
                val lock = value.writeLock()
                if (lock.tryLock() || lock.tryLock(timeout, unit)) {
                    success = true
                    println("Locked file $id")
                }
                return@compute value
            }
            return success
        } else {
            throw InvalidParameterException("ERROR: lockId or ownerId is null!")
        }
    }

ReentrantReadWriteLock 卡在解锁状态

ReentrantReadWriteLock gets stuck on unlock

kotlin

concurrency

deadlock

reentrantreadwritelock