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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain. Use, modify, and |
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* redistribute this code in any way without acknowledgement. |
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*/ |
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|
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package java.util.concurrent; |
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import java.util.concurrent.atomic.*; |
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import java.util.Date; |
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|
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|
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/** |
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* A reentrant mutual exclusion lock. |
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* <p><tt>ReentrantLock</tt> defines a stand-alone {@link Lock} class with |
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* the same basic behavior and semantics as the implicit |
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* monitor lock accessed by the use of <tt>synchronized</tt> methods and |
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* statements, but without the forced block-structured locking and unlocking |
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* that occurs with <tt>synchronized</tt> methods and |
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* statements. |
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* |
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* <p>The order in which blocked threads are granted the lock is not |
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* specified. |
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* |
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* <p>If you want a non-reentrant mutual exclusion lock then it is a simple |
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* matter to use a reentrant lock in a non-reentrant way by ensuring that |
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* the lock is not held by the current thread prior to locking. |
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* See {@link #getHoldCount} for a way to check this. |
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* |
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* |
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* <p><tt>ReentrantLock</tt> instances are intended to be used primarily |
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* in before/after constructions such as: |
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* |
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* <pre> |
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* class X { |
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* ReentrantLock lock; |
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* // ... |
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* |
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* public void m() { |
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* lock.lock(); // block until condition holds |
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* try { |
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* // ... method body |
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* } finally { |
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* lock.unlock() |
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* } |
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* } |
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* } |
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* </pre> |
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* |
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* <p>This class supports the interruption of lock acquisition and provides a |
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* {@link #newCondition Condition} implementation that supports the |
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* interruption of thread suspension. |
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* |
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* <p>Except where noted, passing a <tt>null</tt> value for any parameter |
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* will result in a {@link NullPointerException} being thrown. |
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* |
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* |
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* |
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* @since 1.5 |
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* @spec JSR-166 |
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* @revised $Date: 2003/06/06 17:51:49 $ |
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* @editor $Author: dl $ |
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* @author Doug Lea |
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* |
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* @fixme (1) We need a non-nested example to motivate this |
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* @fixme (2) Describe performance aspects of interruptible locking for the RI |
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**/ |
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public class ReentrantLock extends ReentrantLockQueueNode implements Lock, java.io.Serializable { |
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/* |
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This is a fastpath/slowpath algorithm. |
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|
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* The basic algorithm looks like this, ignoring reentrance, |
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cancellation, timeouts, error checking etc: |
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Lock: |
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if (!fair && casOwner(null, currentThread)) // fastpath |
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return; |
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node = create and enq a wait node; |
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for (;;) { |
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if (node is first on queue) { |
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if (casOwner(null, currentThread)) { |
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deq(node); |
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return; |
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} |
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} |
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park(currentThread); |
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} |
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|
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Unlock: |
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owner = null; // volatile assignment |
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h = first node on queue; |
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if (h != null) unpark(h's successor's thread); |
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|
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* The fast path uses one atomic CAS operation, plus one |
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StoreLoad barrier per lock/unlock pair. The "owner" field is |
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handled as a simple spinlock. To lock, the owner field is set |
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to current thread using conditional atomic update. To unlock, |
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the owner field is set to null, checking if anyone needs |
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waking up, if so doing so. Recursive locks/unlocks instead |
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increment/decrement recursion field. |
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|
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* By default, contended locks use a kind of "greedy" / |
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"renouncement" / barging / convoy-avoidance strategy: When a |
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lock is released, a waiting thread is signalled so that it can |
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(re)contend for the lock. It might lose and thus need to |
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rewait. This strategy has much higher throughput than |
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"directed handoff" because it reduces blocking of running |
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threads, but poorer fairness. The wait queue is FIFO, but |
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newly entering threads can barge ahead and grab lock before |
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woken waiters, so acquires are not strictly FIFO, and transfer |
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is not deterministically fair. It is probablistically fair |
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though. Earlier queued threads are allowed to recontend before |
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later queued threads, and each recontention has an unbiased |
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chance to succeed. |
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|
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* The base class also sets up support for FairReentrantLock |
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subclass, that differs only in that barging is disabled when |
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there is contention, so locks proceed FIFO. There can be |
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some races in detecting contention, but it is still FIFO from |
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a definable (although complicated to describe) single point, |
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so qualifies as a FIFO lock. |
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|
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* The wait queue is a variant of a "CLH" (Craig, Landin, and |
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Hagersten) lock. CLH locks are normally used for spinlocks. |
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We instead use them for blocking locks, but use the same basic |
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tactic of holding the information about whether a thread is |
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released (i.e, eligible to contend for ownership lock) in the |
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predecessor of each node. A node waits until its predecessor |
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says it is released. It is signalled when its predecessor |
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releases the lock. Each node of the queue otherwise serves as |
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a specific-notification-style monitor holding a single waiting |
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thread. |
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|
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To enqueue into a CLH lock, you atomically splice it in as new |
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tail. To dequeue, you just set the head field. |
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|
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+------+ prev +-----+ +-----+ |
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head | | <---- | | <---- | | tail |
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+------+ +-----+ +-----+ |
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|
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The great thing about CLH Locks is that insertion into a CLH |
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queue requires only a single atomic operation on "tail", so |
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there is a simple atomic point of demarcation from unqueued to |
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queued. Similarly, dequeing involves only updating the |
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"head". However, it takes a bit more work for nodes to |
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determine who their successors are, in part to deal with |
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possible cancellation due to timeouts and interrupts. |
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|
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The "prev" links (not used in original CLH locks), are mainly |
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needed to handle cancellation. If a node is cancelled, its |
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successor must be relinked to a non-cancelled predecessor. For |
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explanation in the case of spin locks, see the papers by Scott |
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& Scherer at |
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http://www.cs.rochester.edu/u/scott/synchronization/ |
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|
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Being first in the queue does not mean that you have the lock, |
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only that you may contend for it (by CAS'ing owner field). So |
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the currently released contender thread may need to rewait. |
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|
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We also use "next" links to implement blocking mechanics. The |
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thread id for each node is kept in its node, so a predecessor |
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signals the next node to wake up by traversing next link to |
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determine which thread it is. Determination of successor must |
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avoid races with newly queued nodes to set the "next" fields |
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of their predecessors. This is solved by checking backwards |
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from the atomically updated "tail" when a node's successor |
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appears to be null. |
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|
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Cancellation introduces some conservatism to the basic |
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algorithm of these. Since we must poll for cancellation of |
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other nodes, we can miss noticing whether a cancelled node is |
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ahead or behind us. This is dealt with by always unparking |
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successors upon cancellation, and not letting them park again |
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(by saturating release counts) until they stabilize on a new |
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predecessor. |
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|
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* Threads waiting on Conditions use the same kind of nodes, but |
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only need to link them in simple (non-concurrent) linked |
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queues because they are only accessed when lock is held. To |
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wait, a thread makes a node inserted into a condition queue. |
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Upon signal, the node is transferred to the lock queue. |
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Special values of releaseStatus fields are used to mark which queue a |
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node is on. |
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|
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* All suspension and resumption of threads uses the JSR166 |
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native park/unpark API. These are safe versions of |
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suspend/resume (plus timeout support) that internally avoid |
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the intrinsic race problems with suspend/resume: Park suspends |
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if not preceded by an unpark. Unpark resumes if waiting, else |
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causes next park not to suspend. While safe and efficient, |
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these are not general-purpose public operations because we do |
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not want code outside this package to randomly call these |
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methods -- parks and unparks should be matched up. (It is OK |
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to have more unparks than unparks, but it causes threads to |
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spuriously wake up. So minimizing excessive unparks is a |
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performance concern.) |
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|
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* The ReentrantLock class extends package-private |
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ReentrantLockQueueNode class as an expedient and efficient |
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(although slightly sleazy) solution to serialization and |
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initialization problems. |
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*/ |
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|
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/* |
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Note that all fields are transient and defined in a way that |
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deserialized locks are in initial unlocked state. |
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*/ |
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|
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/** |
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* Creates an instance of <tt>ReentrantLock</tt>. |
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*/ |
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public ReentrantLock() { } |
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|
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/** |
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* Current owner of lock, or null iff the lock is free. Acquired |
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* only using CAS. |
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*/ |
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private transient volatile Thread owner; |
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|
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/** Number of recursive acquires. Note: total holds = recursions+1 */ |
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private transient int recursions; |
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|
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/** Head of the wait queue, initialized to point to self as dummy node */ |
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private transient volatile ReentrantLockQueueNode head = this; |
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|
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/** Tail of the wait queue, initialized to point to self as dummy node */ |
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private transient volatile ReentrantLockQueueNode tail = this; |
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|
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// Atomics support |
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|
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private final static AtomicReferenceFieldUpdater<ReentrantLock, Thread> ownerUpdater = new AtomicReferenceFieldUpdater<ReentrantLock, Thread>(new ReentrantLock[0], new Thread[0], "owner"); |
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private final static AtomicReferenceFieldUpdater<ReentrantLock, ReentrantLockQueueNode> tailUpdater = new AtomicReferenceFieldUpdater<ReentrantLock, ReentrantLockQueueNode>(new ReentrantLock[0], new ReentrantLockQueueNode[0], "tail"); |
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private final static AtomicReferenceFieldUpdater<ReentrantLock, ReentrantLockQueueNode> headUpdater = new AtomicReferenceFieldUpdater<ReentrantLock, ReentrantLockQueueNode>(new ReentrantLock[0], new ReentrantLockQueueNode[0], "head"); |
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private final static AtomicIntegerFieldUpdater<ReentrantLockQueueNode> releaseStatusUpdater = |
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new AtomicIntegerFieldUpdater<ReentrantLockQueueNode>(new ReentrantLockQueueNode[0], "releaseStatus"); |
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|
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private boolean acquireOwner(Thread current) { |
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return ownerUpdater.compareAndSet(this, null, current); |
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} |
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|
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private boolean casTail(ReentrantLockQueueNode cmp, ReentrantLockQueueNode val) { |
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return tailUpdater.compareAndSet(this, cmp, val); |
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} |
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|
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private boolean casHead(ReentrantLockQueueNode cmp, ReentrantLockQueueNode val) { |
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return headUpdater.compareAndSet(this, cmp, val); |
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} |
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|
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// casReleaseStatus non-private because also accessed by Conditions |
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final boolean casReleaseStatus(ReentrantLockQueueNode node, int cmp, int val) { |
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return releaseStatusUpdater.compareAndSet(node, cmp, val); |
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} |
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|
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|
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/** |
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* Special value for releaseStatus indicating that node is cancelled. |
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* Must be a large positive number. |
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*/ |
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static private final int CANCELLED = Integer.MAX_VALUE; |
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|
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/** |
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* Special value for node releaseStatus indicating that node is on a |
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* condition queue. Must be large negative number. |
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*/ |
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static private final int ON_CONDITION_QUEUE = Integer.MIN_VALUE; |
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|
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/** |
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* Special value for node releaseStatus indicating that node is in |
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* process of transfer. Must be negative and greater than |
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* ON_CONDITION_QUEUE. |
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*/ |
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static private final int TRANSFERRING = ON_CONDITION_QUEUE+1; |
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|
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|
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/** |
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* Utility to throw an exception if lock not held by given thread. |
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*/ |
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final void checkOwner(Thread current) { |
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if (current != owner) |
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throw new IllegalMonitorStateException(); |
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} |
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|
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/** |
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* Return whether lock wait queue is empty |
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*/ |
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final boolean queueEmpty() { |
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ReentrantLockQueueNode h = head; // force order of the volatile reads |
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return h == tail; |
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} |
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|
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/** |
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* Insert node into queue. Return predecessor. |
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*/ |
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private ReentrantLockQueueNode enq(ReentrantLockQueueNode node) { |
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for (;;) { |
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ReentrantLockQueueNode p = tail; |
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node.prev = p; // prev must be valid before/upon CAS |
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if (casTail(p, node)) { |
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p.next = node; // Note: next field assignment lags CAS |
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return p; |
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} |
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} |
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} |
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|
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/** |
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* Return true if it is OK to take fast path to lock. |
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* Overridden in FairReentrantLock. |
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*/ |
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boolean canBarge() { |
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return true; |
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} |
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|
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/* |
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* policy/status values for main lock routine. A bit ugly but |
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* worth it to factor the common lock code across the different |
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* policies for suppressing or throwing IE and indicating |
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* timeouts. |
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*/ |
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|
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/** |
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* As param, suppress interrupts. |
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* As return, lock was successfully obtained. |
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*/ |
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static final int NO_INTERRUPT = 0; |
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|
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/** |
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* As param, allow interrupts. |
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* As return, lock was not obtained because thread was interrupted. |
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*/ |
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static final int INTERRUPT = -1; |
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|
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|
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/** |
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* As param, use timed waits. |
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* As return, lock was not obtained because timeout was exceeded. |
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*/ |
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static final int TIMEOUT = -2; |
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|
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/** |
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* Main locking code, parameterized across different policies. |
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* @param current current thread |
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* @param node its wait-node, it is alread exists; else null in |
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* which case it is created, |
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* @param policy -- NO_INTERRUPT, INTERRUPT, or TIMEOUT |
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* thread interrupted |
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* @param nanos time to wait, or zero if untimed |
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* @return NO_INTERRUPT on success, else INTERRUPT, or TIMEOUT |
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*/ |
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final int doLock(Thread current, |
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ReentrantLockQueueNode node, |
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int policy, |
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long nanos) { |
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|
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int status = NO_INTERRUPT; |
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|
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/* |
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* Bypass queueing if a recursive lock |
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*/ |
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if (owner == current) { |
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++recursions; |
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return status; |
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} |
361 |
|
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|
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long lastTime = 0; // for adjusting timeouts, below |
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|
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/* |
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* p is our predecessor node, that holds releaseStatus giving |
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* permission to try to obtain lock if we are first in queue. |
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*/ |
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ReentrantLockQueueNode p; |
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|
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/* |
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* Create and enqueue node if not already created. Nodes |
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* transferred from condition queues will already be created |
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* and queued. |
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*/ |
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if (node == null) { |
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node = new ReentrantLockQueueNode(current); |
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p = enq(node); |
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} |
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else |
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p = node.prev; |
382 |
|
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/* |
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* Repeatedly try to get ownership if first in queue, else |
385 |
* block. |
386 |
*/ |
387 |
for (;;) { |
388 |
/* |
389 |
* If we are the first thread in queue, try to get the |
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* lock. (We must not try to get lock if we are not |
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* first.) Note that if we become first after p == head |
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* check, all is well -- we can be sure an unlocking |
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* thread will signal us. |
394 |
*/ |
395 |
|
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if (p == head && acquireOwner(current)) { |
397 |
/* |
398 |
* status can be INTERRUPT here if policy is |
399 |
* NO_INTERRUPT but thread was interrupted while |
400 |
* waiting, in which case we must reset status upon |
401 |
* return. |
402 |
*/ |
403 |
if (status == INTERRUPT) |
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current.interrupt(); |
405 |
|
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p.next = null; // clear for GC and to suppress signals |
407 |
node.thread = null; |
408 |
node.prev = null; |
409 |
head = node; |
410 |
return NO_INTERRUPT; |
411 |
} |
412 |
|
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int releaseStatus = p.releaseStatus; |
414 |
|
415 |
/* |
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* If our predecessor was cancelled, use its predecessor. |
417 |
* There will always be a non-cancelled one somewhere |
418 |
* because head node is never cancelled, so at worst we |
419 |
* will hit it. (Note that because head is never |
420 |
* cancelled, we can perform this check after trying to |
421 |
* acquire ownership). |
422 |
*/ |
423 |
if (releaseStatus == CANCELLED) { |
424 |
node.prev = p = p.prev; |
425 |
} |
426 |
|
427 |
/* |
428 |
* Wait if we are not not first in queue, or if we are |
429 |
* first, we have tried to acquire owner and failed since |
430 |
* either entry or last release. (Note that releaseStatus can |
431 |
* already be less than zero if we spuriously returned |
432 |
* from a previous park or got new a predecessor due to |
433 |
* cancellation.) |
434 |
* |
435 |
* We also don't wait if atomic decrement of releaseStatus |
436 |
* fails. We just continue main loop on failure to |
437 |
* atomically update releaseStatus because interference |
438 |
* causing failure is almost surely due to someone |
439 |
* releasing us anyway. |
440 |
* |
441 |
* Each wait consumes all available releases. Normally |
442 |
* there is only one anyway because release doesn't bother |
443 |
* incrementing if already positive. |
444 |
* |
445 |
*/ |
446 |
else if (casReleaseStatus(p, releaseStatus, (releaseStatus > 0)? 0 : -1) && releaseStatus <= 0) { |
447 |
|
448 |
// Update and check timeout value |
449 |
if (nanos > 0) { |
450 |
long now = TimeUnit.nanoTime(); |
451 |
if (lastTime != 0) { |
452 |
nanos -= now - lastTime; |
453 |
if (nanos <= 0) |
454 |
status = TIMEOUT; |
455 |
} |
456 |
lastTime = now; |
457 |
} |
458 |
|
459 |
if (status != TIMEOUT) |
460 |
JSR166Support.park(false, nanos); |
461 |
|
462 |
if (Thread.interrupted()) |
463 |
status = INTERRUPT; |
464 |
|
465 |
if (status != NO_INTERRUPT && policy != NO_INTERRUPT) { |
466 |
node.thread = null; // disable signals |
467 |
releaseStatusUpdater.set(node, CANCELLED); // don't need CAS here |
468 |
signalSuccessor(node); |
469 |
return status; |
470 |
} |
471 |
} |
472 |
} |
473 |
} |
474 |
|
475 |
/** |
476 |
* Wake up node's successor, if one exists |
477 |
*/ |
478 |
private void signalSuccessor(ReentrantLockQueueNode node) { |
479 |
/* |
480 |
* Find successor -- normally just node.next. |
481 |
*/ |
482 |
ReentrantLockQueueNode s = node.next; |
483 |
|
484 |
/* |
485 |
* if s is cancelled, traverse through next's. |
486 |
*/ |
487 |
|
488 |
while (s != null && s.releaseStatus == CANCELLED) { |
489 |
node = s; |
490 |
s = s.next; |
491 |
} |
492 |
|
493 |
/* |
494 |
* If successor appears to be null, check to see if a newly |
495 |
* queued node is successor by starting at tail and working |
496 |
* backwards. If so, help out the enqueing thread by setting |
497 |
* next field. We don't expect this loop to trigger often, |
498 |
* and hardly ever to iterate. |
499 |
*/ |
500 |
|
501 |
if (s == null) { |
502 |
ReentrantLockQueueNode t = tail; |
503 |
for (;;) { |
504 |
/* |
505 |
* If t == node, there is no successor. |
506 |
*/ |
507 |
if (t == node) |
508 |
return; |
509 |
|
510 |
ReentrantLockQueueNode tp = t.prev; |
511 |
|
512 |
/* |
513 |
* t's predecessor is null if we are lagging so far |
514 |
* behind the actions of other nodes/threads that an |
515 |
* intervening head.prev was nulled by some |
516 |
* non-cancelled successor of node. In which case, |
517 |
* there's no live successor. |
518 |
*/ |
519 |
|
520 |
if (tp == null) |
521 |
return; |
522 |
|
523 |
/* |
524 |
* If we find successor, we can do the assignment to |
525 |
* next (don't even need CAS) on behalf of enqueuing |
526 |
* thread. At worst we will stall now and lag behind |
527 |
* both the setting and the later clearing of next |
528 |
* field. But if so, we will reattach an internal link |
529 |
* in soon-to-be unreachable set of nodes, so no harm |
530 |
* done. |
531 |
*/ |
532 |
|
533 |
if (tp == node) { |
534 |
node.next = s = t; |
535 |
break; |
536 |
} |
537 |
|
538 |
t = tp; |
539 |
|
540 |
/* |
541 |
* Before iterating, check to see if link has |
542 |
* appeared. |
543 |
*/ |
544 |
ReentrantLockQueueNode n = node.next; |
545 |
if (n != null) { |
546 |
s = n; |
547 |
break; |
548 |
} |
549 |
} |
550 |
} |
551 |
|
552 |
Thread thr = s.thread; |
553 |
if (thr != null && thr != owner) // don't bother signalling if has lock |
554 |
JSR166Support.unpark(thr); |
555 |
} |
556 |
|
557 |
|
558 |
/** |
559 |
* Increment releaseStatus and signal next thread in queue if one |
560 |
* exists and is waiting. Called only by unlock. This code is split |
561 |
* out from unlock to encourage inlining of non-contended cases. |
562 |
*/ |
563 |
private void releaseFirst() { |
564 |
for (;;) { |
565 |
ReentrantLockQueueNode h = head; |
566 |
if (h == tail) // No successor |
567 |
return; |
568 |
|
569 |
int c = h.releaseStatus; |
570 |
if (c > 0) // Don't need signal if already positive |
571 |
return; |
572 |
if (owner != null) // Don't bother if some thread got lock |
573 |
return; |
574 |
if (casReleaseStatus(h, c, (c < 0)? 0 : 1)) { // saturate at 1 |
575 |
if (c < 0) |
576 |
signalSuccessor(h); |
577 |
return; |
578 |
} |
579 |
} |
580 |
} |
581 |
|
582 |
/** |
583 |
* Attempts to release this lock. <p>If the current thread is the |
584 |
* holder of this lock then the hold count is decremented. If the |
585 |
* hold count is now zero then the lock is released. <p>If the |
586 |
* current thread is not the holder of this lock then {@link |
587 |
* IllegalMonitorStateException} is thrown. |
588 |
* @throws IllegalMonitorStateException if the current thread does not |
589 |
* hold this lock. |
590 |
*/ |
591 |
public void unlock() { |
592 |
checkOwner(Thread.currentThread()); |
593 |
|
594 |
if (recursions > 0) { |
595 |
--recursions; |
596 |
return; |
597 |
} |
598 |
|
599 |
ownerUpdater.set(this, null); |
600 |
if (tail != this) // don't bother if never contended |
601 |
releaseFirst(); |
602 |
} |
603 |
|
604 |
/** |
605 |
* Acquire the lock. |
606 |
* <p>Acquires the lock if it is not held be another thread and returns |
607 |
* immediately, setting the lock hold count to one. |
608 |
* <p>If the current thread |
609 |
* already holds the lock then the hold count is incremented by one and |
610 |
* the method returns immediately. |
611 |
* <p>If the lock is held by another thread then the |
612 |
* the current thread thread becomes disabled for thread scheduling |
613 |
* purposes and lies dormant until the lock has been acquired |
614 |
* at which time the lock hold count is set to one. |
615 |
*/ |
616 |
public void lock() { |
617 |
Thread current = Thread.currentThread(); |
618 |
if (canBarge() && acquireOwner(current)) |
619 |
return; |
620 |
int stat = doLock(current, null, NO_INTERRUPT, 0); |
621 |
// assert stat == NO_INTERRUPT; |
622 |
} |
623 |
|
624 |
/** |
625 |
* Acquires the lock unless the current thread is |
626 |
* {@link Thread#interrupt interrupted}. |
627 |
* <p>Acquires the lock if it is not held by another thread and returns |
628 |
* immediately, setting the lock hold count to one. |
629 |
* <p>If the current thread already holds this lock then the hold count |
630 |
* is incremented by one and the method returns immediately. |
631 |
* <p>If the lock is held by another thread then the |
632 |
* the current thread becomes disabled for thread scheduling |
633 |
* purposes and lies dormant until one of two things happens: |
634 |
* <ul> |
635 |
* <li> The lock is acquired by the current thread; or |
636 |
* <li> Some other thread {@link Thread#interrupt interrupts} the current |
637 |
* thread. |
638 |
* </ul> |
639 |
* <p>If the lock is acquired by the current thread then the lock hold |
640 |
* count is set to one. |
641 |
* <p>If the current thread: |
642 |
* <ul> |
643 |
* <li>has its interrupted status set on entry to this method; or |
644 |
* <li>is {@link Thread#interrupt interrupted} while waiting to acquire |
645 |
* the lock, |
646 |
* </ul> |
647 |
* then {@link InterruptedException} is thrown and the current thread's |
648 |
* interrupted status is cleared. As this method is an explicit |
649 |
* interruption point, preference is given to responding to the interrupt |
650 |
* over reentrant acquisition of the lock. |
651 |
* |
652 |
* |
653 |
* @throws InterruptedException if the current thread is interrupted |
654 |
*/ |
655 |
public void lockInterruptibly() throws InterruptedException { |
656 |
Thread current = Thread.currentThread(); |
657 |
if (Thread.interrupted()) |
658 |
throw new InterruptedException(); |
659 |
if (canBarge() && acquireOwner(current)) |
660 |
return; |
661 |
if (doLock(current, null, INTERRUPT, 0) == INTERRUPT) |
662 |
throw new InterruptedException(); |
663 |
} |
664 |
|
665 |
/** |
666 |
* Acquires the lock only if it is not held by another thread at the time |
667 |
* of invocation. |
668 |
* <p>Acquires the lock if it is not held by another thread and returns |
669 |
* immediately with the value <tt>true</tt>, setting the lock hold count |
670 |
* to one. |
671 |
* <p> If the current thread |
672 |
* already holds this lock then the hold count is incremented by one and |
673 |
* the method returns <tt>true</tt>. |
674 |
* <p>If the lock is held by another thread then this method will return |
675 |
* immediately with the value <tt>false</tt>. |
676 |
* |
677 |
* @return <tt>true</tt>if the lock was free and was acquired by the |
678 |
* current thread, or the lock was already held by the current thread; and |
679 |
* <tt>false</tt> otherwise. |
680 |
*/ |
681 |
public boolean tryLock() { |
682 |
Thread current = Thread.currentThread(); |
683 |
if (acquireOwner(current)) |
684 |
return true; |
685 |
if (owner == current) { |
686 |
++recursions; |
687 |
return true; |
688 |
} |
689 |
return false; |
690 |
} |
691 |
|
692 |
/** |
693 |
* |
694 |
* Acquires the lock if it is not held by another thread within the given |
695 |
* waiting time and the current thread has not been interrupted. |
696 |
* <p>Acquires the lock if it is not held by another thread and returns |
697 |
* immediately with the value <tt>true</tt>, setting the lock hold count |
698 |
* to one. |
699 |
* <p> If the current thread |
700 |
* already holds this lock then the hold count is incremented by one and |
701 |
* the method returns <tt>true</tt>. |
702 |
* <p>If the lock is held by another thread then the |
703 |
* the current thread becomes disabled for thread scheduling |
704 |
* purposes and lies dormant until one of three things happens: |
705 |
* <ul> |
706 |
* <li> The lock is acquired by the current thread; or |
707 |
* <li> Some other thread {@link Thread#interrupt interrupts} the current |
708 |
* thread; or |
709 |
* <li> The specified waiting time elapses |
710 |
* </ul> |
711 |
* <p>If the lock is acquired then the value <tt>true</tt> is returned and |
712 |
* the lock hold count is set to one. |
713 |
* <p>If the current thread: |
714 |
* <ul> |
715 |
* <li>has its interrupted status set on entry to this method; or |
716 |
* <li>is {@link Thread#interrupt interrupted} before acquiring |
717 |
* the lock, |
718 |
* </ul> |
719 |
* then {@link InterruptedException} is thrown and the current thread's |
720 |
* interrupted status is cleared. As this method is an explicit |
721 |
* interruption point, preference is given to responding to the interrupt |
722 |
* over reentrant acquisition of the lock. |
723 |
* <p>If the specified waiting time elapses then the value <tt>false</tt> |
724 |
* is returned. |
725 |
* <p>The given waiting time is a best-effort lower bound. If the time is |
726 |
* less than or equal to zero, the method will not wait at all. |
727 |
* |
728 |
* |
729 |
* @return <tt>true</tt> if the lock was free and was acquired by the |
730 |
* current thread, or the lock was already held by the current thread; and |
731 |
* <tt>false</tt> if the waiting time elapsed before the lock could be |
732 |
* acquired. |
733 |
* |
734 |
* @throws InterruptedException if the current thread is interrupted |
735 |
*/ |
736 |
public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { |
737 |
if (unit == null) |
738 |
throw new NullPointerException(); |
739 |
if (Thread.interrupted()) |
740 |
throw new InterruptedException(); |
741 |
Thread current = Thread.currentThread(); |
742 |
if (canBarge() && acquireOwner(current)) |
743 |
return true; |
744 |
if (owner == current) { |
745 |
++recursions; |
746 |
return true; |
747 |
} |
748 |
if (timeout <= 0) |
749 |
return false; |
750 |
int stat = doLock(current, null, TIMEOUT, unit.toNanos(timeout)); |
751 |
if (stat == NO_INTERRUPT) |
752 |
return true; |
753 |
if (stat == INTERRUPT) |
754 |
throw new InterruptedException(); |
755 |
assert stat == TIMEOUT; |
756 |
return false; |
757 |
} |
758 |
|
759 |
|
760 |
/** |
761 |
* Queries the number of holds on this lock by the current thread. |
762 |
* <p>A thread has a hold on a lock for each lock action that is not |
763 |
* matched by an unlock action. |
764 |
* <p>The hold count information is typically only used for testing and |
765 |
* debugging purposes. For example, if a certain section of code should |
766 |
* not be entered with the lock already held then we can assert that |
767 |
* fact: |
768 |
* <pre> |
769 |
* class X { |
770 |
* ReentrantLock lock = new ReentrantLock(); |
771 |
* // ... |
772 |
* |
773 |
* public void m() { |
774 |
* assert lock.getHoldCount() == 0; |
775 |
* lock.lock(); |
776 |
* try { |
777 |
* // ... method body |
778 |
* } finally { |
779 |
* lock.unlock() |
780 |
* } |
781 |
* } |
782 |
* } |
783 |
* </pre> |
784 |
* |
785 |
* @return the number of holds on this lock by current thread, |
786 |
* or zero if this lock is not held by current thread. |
787 |
**/ |
788 |
public int getHoldCount() { |
789 |
return (owner == Thread.currentThread()) ? recursions + 1 : 0; |
790 |
} |
791 |
|
792 |
/** |
793 |
* Queries if this lock is held by the current thread. |
794 |
* <p>Analogous to the {@link Thread#holdsLock} method for built-in |
795 |
* monitor locks, this method is typically used for debugging and |
796 |
* testing. For example, a method that should only be called while |
797 |
* a lock is held can assert that this is the case: |
798 |
* <pre> |
799 |
* class X { |
800 |
* ReentrantLock lock = new ReentrantLock(); |
801 |
* // ... |
802 |
* |
803 |
* public void m() { |
804 |
* assert lock.isHeldByCurrentThread(); |
805 |
* // ... method body |
806 |
* } |
807 |
* } |
808 |
* </pre> |
809 |
* |
810 |
* @return <tt>true</tt> if current thread holds this lock and |
811 |
* <tt>false</tt> otherwise. |
812 |
**/ |
813 |
public boolean isHeldByCurrentThread() { |
814 |
return (owner == Thread.currentThread()); |
815 |
} |
816 |
|
817 |
|
818 |
/** |
819 |
* Queries if this lock is held by any thread. THis method is |
820 |
* designed for use in monitoring, not for synchronization control. |
821 |
* @return <tt>true</tt> if any thread holds this lock and |
822 |
* <tt>false</tt> otherwise. |
823 |
**/ |
824 |
public boolean isLocked() { |
825 |
return owner != null; |
826 |
} |
827 |
|
828 |
/** |
829 |
* Reconstitute by resetting head and tail to point back to the lock. |
830 |
*/ |
831 |
private void readObject(java.io.ObjectInputStream s) |
832 |
throws java.io.IOException, ClassNotFoundException { |
833 |
s.defaultReadObject(); |
834 |
head = tail = this; |
835 |
} |
836 |
|
837 |
/** |
838 |
* Returns a {@link Condition} instance for use with this |
839 |
* {@link Lock} instance. |
840 |
* |
841 |
* <p>The returned {@link Condition} instance has the same behavior and |
842 |
* usage |
843 |
* restrictions with this lock as the {@link Object} monitor methods |
844 |
* ({@link Object#wait() wait}, {@link Object#notify notify}, and |
845 |
* {@link Object#notifyAll notifyAll}) have with the built-in monitor |
846 |
* lock: |
847 |
* <ul> |
848 |
* <li>If this lock is not held when any of the {@link Condition} |
849 |
* {@link Condition#await() waiting} or {@link Condition#signal signalling} |
850 |
* methods are called, then an {@link IllegalMonitorStateException} is |
851 |
* thrown. |
852 |
* <li>When the condition {@link Condition#await() waiting} methods are |
853 |
* called the lock is released and before they return the lock is |
854 |
* reacquired and the lock hold count restored to what it was when the |
855 |
* method was called. |
856 |
* <li>If a thread is {@link Thread#interrupt interrupted} while waiting |
857 |
* then the wait will terminate, an {@link InterruptedException} will be |
858 |
* thrown, and the thread's interrupted status will be cleared. |
859 |
* <li>The order in which waiting threads are signalled is not specified. |
860 |
* <li>The order in which threads returning from a wait, and threads trying |
861 |
* to acquire the lock, are granted the lock, is not specified. |
862 |
* </ul> |
863 |
*/ |
864 |
public Condition newCondition() { |
865 |
return new ReentrantLockConditionObject(); |
866 |
} |
867 |
|
868 |
// Helper methods for Conditions |
869 |
|
870 |
/** |
871 |
* Return true if a node, always one that was initially placed on |
872 |
* a condition queue, is off the condition queue (and thus, |
873 |
* normally is now on lock queue.) |
874 |
*/ |
875 |
boolean isOffConditionQueue(ReentrantLockQueueNode w) { |
876 |
return w.releaseStatus > TRANSFERRING; |
877 |
} |
878 |
|
879 |
/** |
880 |
* Transfer a node from condition queue onto lock queue. |
881 |
* Return true if successful (i.e., node not cancelled) |
882 |
*/ |
883 |
final boolean transferToLockQueue(ReentrantLockQueueNode node) { |
884 |
/* |
885 |
* Atomically change status to TRANSFERRING to avoid races |
886 |
* with cancelling waiters. We use a special value that causes |
887 |
* any waiters spuriously waking up to re-park until the node |
888 |
* has been placed on lock queue. |
889 |
*/ |
890 |
if (!casReleaseStatus(node, ON_CONDITION_QUEUE, TRANSFERRING)) |
891 |
return false; |
892 |
|
893 |
/* |
894 |
* Splice onto queue |
895 |
*/ |
896 |
ReentrantLockQueueNode p = enq(node); |
897 |
|
898 |
/* |
899 |
* Establish normal lock-queue releaseStatus for node. The |
900 |
* CAS can fail if node already was involved in a cancellation |
901 |
* on lock-queue, in which case we signal to be sure. |
902 |
*/ |
903 |
if (!casReleaseStatus(node, TRANSFERRING, 0)) |
904 |
signalSuccessor(node); |
905 |
|
906 |
/* |
907 |
* Ensure releaseStatus of predecessor is negative to indicate |
908 |
* that thread is (probably) waiting. If attempt to set releaseStatus |
909 |
* fails or is pred is/becomes cancelled, wake up successor |
910 |
* (which will ordinarily be "node") to resynch. |
911 |
*/ |
912 |
|
913 |
for (;;) { |
914 |
int c = p.releaseStatus; |
915 |
if (c < 0 || (c != CANCELLED && casReleaseStatus(p, c, -1))) |
916 |
break; |
917 |
signalSuccessor(p); |
918 |
if (c == CANCELLED) |
919 |
break; |
920 |
} |
921 |
|
922 |
return true; |
923 |
} |
924 |
|
925 |
/** |
926 |
* Hook method used by ReentrantReadWriteLock. Called |
927 |
* before unlocking lock to enter wait. |
928 |
*/ |
929 |
void beforeWait() { } |
930 |
|
931 |
|
932 |
/** |
933 |
* Hook method used by ReentrantReadWriteLock. Called |
934 |
* after locking lock after exiting wait. |
935 |
*/ |
936 |
void afterWait() { } |
937 |
|
938 |
private class ReentrantLockConditionObject implements Condition, java.io.Serializable { |
939 |
/* |
940 |
* Because condition queues are accessed only when locks are |
941 |
* already held, we just need a simple linked queue to hold |
942 |
* nodes while they are waiting on conditions. They are then |
943 |
* transferred to the lock queue to re-acquire locks. |
944 |
*/ |
945 |
|
946 |
/** |
947 |
* First node of condition queue. |
948 |
*/ |
949 |
private transient ReentrantLockQueueNode firstWaiter; |
950 |
|
951 |
/** |
952 |
* Last node of condition queue. |
953 |
*/ |
954 |
private transient ReentrantLockQueueNode lastWaiter; |
955 |
|
956 |
/** |
957 |
* Basic linked queue insertion. |
958 |
*/ |
959 |
private ReentrantLockQueueNode addWaiter(Thread current) { |
960 |
ReentrantLockQueueNode w = new ReentrantLockQueueNode(current); |
961 |
w.releaseStatus = ON_CONDITION_QUEUE; |
962 |
if (lastWaiter == null) |
963 |
firstWaiter = lastWaiter = w; |
964 |
else { |
965 |
ReentrantLockQueueNode t = lastWaiter; |
966 |
lastWaiter = w; |
967 |
t.next = w; |
968 |
} |
969 |
return w; |
970 |
} |
971 |
|
972 |
/** |
973 |
* Main code for signal. Dequeue and transfer nodes until hit |
974 |
* non-cancelled one or null. Split out from signal to |
975 |
* encourage compilers to inline the case of no waiters. |
976 |
*/ |
977 |
private void doSignal(ReentrantLockQueueNode first) { |
978 |
do { |
979 |
if ( (firstWaiter = first.next) == null) |
980 |
lastWaiter = null; |
981 |
first.next = null; |
982 |
if (transferToLockQueue(first)) |
983 |
return; |
984 |
first = firstWaiter; |
985 |
} while (first != null); |
986 |
} |
987 |
|
988 |
public void signal() { |
989 |
checkOwner(Thread.currentThread()); |
990 |
ReentrantLockQueueNode w = firstWaiter; |
991 |
if (w != null) |
992 |
doSignal(w); |
993 |
} |
994 |
|
995 |
public void signalAll() { |
996 |
checkOwner(Thread.currentThread()); |
997 |
// Pull off list all at once and traverse. |
998 |
ReentrantLockQueueNode w = firstWaiter; |
999 |
if (w != null) { |
1000 |
lastWaiter = firstWaiter = null; |
1001 |
do { |
1002 |
ReentrantLockQueueNode n = w.next; |
1003 |
w.next = null; |
1004 |
transferToLockQueue(w); |
1005 |
w = n; |
1006 |
} while (w != null); |
1007 |
} |
1008 |
} |
1009 |
|
1010 |
/* |
1011 |
* Various flavors of wait. Each almost the same, but |
1012 |
* annoyingly different and no nice way to factor common code. |
1013 |
*/ |
1014 |
|
1015 |
public void await() throws InterruptedException { |
1016 |
Thread current = Thread.currentThread(); |
1017 |
checkOwner(current); |
1018 |
|
1019 |
ReentrantLockQueueNode w = addWaiter(current); |
1020 |
beforeWait(); |
1021 |
int recs = recursions; |
1022 |
unlock(); |
1023 |
|
1024 |
boolean wasInterrupted = false; |
1025 |
|
1026 |
while (!isOffConditionQueue(w)) { |
1027 |
JSR166Support.park(false, 0); |
1028 |
if (Thread.interrupted()) { |
1029 |
wasInterrupted = true; |
1030 |
if (casReleaseStatus(w, ON_CONDITION_QUEUE, CANCELLED)) { |
1031 |
w.thread = null; |
1032 |
w = null; |
1033 |
} |
1034 |
break; |
1035 |
} |
1036 |
} |
1037 |
|
1038 |
/* |
1039 |
* If we exited above loop due to cancellation, then w is |
1040 |
* null, and doLock will make a new lock node for |
1041 |
* us. Otherwise, upon exit, our node is already in the |
1042 |
* lock queue when doLock is called. |
1043 |
*/ |
1044 |
doLock(current, w, NO_INTERRUPT, 0); |
1045 |
|
1046 |
recursions = recs; |
1047 |
afterWait(); |
1048 |
|
1049 |
if (wasInterrupted) |
1050 |
throw new InterruptedException(); |
1051 |
} |
1052 |
|
1053 |
public void awaitUninterruptibly() { |
1054 |
Thread current = Thread.currentThread(); |
1055 |
checkOwner(current); |
1056 |
|
1057 |
ReentrantLockQueueNode w = addWaiter(current); |
1058 |
beforeWait(); |
1059 |
int recs = recursions; |
1060 |
unlock(); |
1061 |
|
1062 |
boolean wasInterrupted = false; |
1063 |
while (!isOffConditionQueue(w)) { |
1064 |
JSR166Support.park(false, 0); |
1065 |
if (Thread.interrupted()) |
1066 |
wasInterrupted = true; |
1067 |
} |
1068 |
|
1069 |
doLock(current, w, NO_INTERRUPT, 0); |
1070 |
recursions = recs; |
1071 |
afterWait(); |
1072 |
// avoid re-interrupts on exit |
1073 |
if (wasInterrupted && !current.isInterrupted()) |
1074 |
current.interrupt(); |
1075 |
} |
1076 |
|
1077 |
|
1078 |
public long awaitNanos(long nanos) throws InterruptedException { |
1079 |
Thread current = Thread.currentThread(); |
1080 |
checkOwner(current); |
1081 |
|
1082 |
ReentrantLockQueueNode w = addWaiter(current); |
1083 |
beforeWait(); |
1084 |
int recs = recursions; |
1085 |
unlock(); |
1086 |
|
1087 |
if (nanos <= 0) nanos = 1; // park arg must be positive |
1088 |
long timeLeft = nanos; |
1089 |
long startTime = TimeUnit.nanoTime(); |
1090 |
boolean wasInterrupted = false; |
1091 |
boolean cancelled = false; |
1092 |
|
1093 |
if (!isOffConditionQueue(w)) { |
1094 |
for (;;) { |
1095 |
JSR166Support.park(false, timeLeft); |
1096 |
if (Thread.interrupted()) |
1097 |
wasInterrupted = true; |
1098 |
else if (isOffConditionQueue(w)) |
1099 |
break; |
1100 |
else |
1101 |
timeLeft = nanos - (TimeUnit.nanoTime() - startTime); |
1102 |
|
1103 |
if (wasInterrupted || timeLeft <= 0) { |
1104 |
if (casReleaseStatus(w, ON_CONDITION_QUEUE, CANCELLED)) { |
1105 |
w.thread = null; |
1106 |
w = null; |
1107 |
} |
1108 |
break; |
1109 |
} |
1110 |
} |
1111 |
} |
1112 |
|
1113 |
doLock(current, w, NO_INTERRUPT, 0); |
1114 |
recursions = recs; |
1115 |
afterWait(); |
1116 |
|
1117 |
if (wasInterrupted) |
1118 |
throw new InterruptedException(); |
1119 |
else if (timeLeft <= 0) |
1120 |
return timeLeft; |
1121 |
else |
1122 |
return nanos - (TimeUnit.nanoTime() - startTime); |
1123 |
} |
1124 |
|
1125 |
public boolean awaitUntil(Date deadline) throws InterruptedException { |
1126 |
Thread current = Thread.currentThread(); |
1127 |
checkOwner(current); |
1128 |
|
1129 |
ReentrantLockQueueNode w = addWaiter(current); |
1130 |
beforeWait(); |
1131 |
int recs = recursions; |
1132 |
unlock(); |
1133 |
|
1134 |
boolean wasInterrupted = false; |
1135 |
boolean cancelled = false; |
1136 |
long abstime = deadline.getTime(); |
1137 |
|
1138 |
if (!isOffConditionQueue(w)) { |
1139 |
for (;;) { |
1140 |
JSR166Support.park(true, abstime); |
1141 |
|
1142 |
boolean timedOut = false; |
1143 |
if (Thread.interrupted()) |
1144 |
wasInterrupted = true; |
1145 |
else if (isOffConditionQueue(w)) |
1146 |
break; |
1147 |
else if (System.currentTimeMillis() <= abstime) |
1148 |
timedOut = true; |
1149 |
|
1150 |
if (wasInterrupted || timedOut) { |
1151 |
if (casReleaseStatus(w, ON_CONDITION_QUEUE, CANCELLED)) { |
1152 |
w.thread = null; |
1153 |
w = null; |
1154 |
} |
1155 |
break; |
1156 |
} |
1157 |
} |
1158 |
} |
1159 |
|
1160 |
doLock(current, w, NO_INTERRUPT, 0); |
1161 |
recursions = recs; |
1162 |
afterWait(); |
1163 |
|
1164 |
if (wasInterrupted) |
1165 |
throw new InterruptedException(); |
1166 |
return !cancelled; |
1167 |
} |
1168 |
|
1169 |
public boolean await(long time, TimeUnit unit) throws InterruptedException { |
1170 |
return awaitNanos(unit.toNanos(time)) > 0; |
1171 |
} |
1172 |
|
1173 |
} |
1174 |
|
1175 |
} |
1176 |
|
1177 |
/** |
1178 |
* Node class for threads waiting for locks. This cannot be nested |
1179 |
* inside ReentrantLock because of Java inheritance circularity rules. |
1180 |
*/ |
1181 |
class ReentrantLockQueueNode { |
1182 |
/** |
1183 |
* Controls whether successor node is allowed to try to obtain |
1184 |
* ownership. Acts as a saturating (in both directions) counting |
1185 |
* semaphore: Upon each wait, the releaseStatus is reduced to zero |
1186 |
* if positive, else reduced to negative, in which case the thread |
1187 |
* will park. The releaseStatus is incremented on each unlock that |
1188 |
* would enable successor thread to obtain lock (succeeding if |
1189 |
* there is no contention). The special value of CANCELLED is used |
1190 |
* to mean that the releaseStatus cannot be either incremented or |
1191 |
* decremented. The special value of ON_CONDITION_QUEUE is used |
1192 |
* when nodes are on conditions queues instead of lock queue, and |
1193 |
* the special value TRANSFERRING is used while signals are in |
1194 |
* progress. |
1195 |
*/ |
1196 |
transient volatile int releaseStatus; |
1197 |
|
1198 |
/** |
1199 |
* Link to predecessor node that current node/thread relies on |
1200 |
* for checking releaseStatus. Assigned once during enqueing, |
1201 |
* and nulled out (for sake of GC) only upon dequeuing. Upon |
1202 |
* cancellation, we do NOT adjust this field, but simply |
1203 |
* traverse through prev's until we hit a non-cancelled node. |
1204 |
* A valid predecessor will always exist because the head node |
1205 |
* is never cancelled. |
1206 |
*/ |
1207 |
transient volatile ReentrantLockQueueNode prev; |
1208 |
|
1209 |
/** |
1210 |
* Link to the successor node that the current node/thread |
1211 |
* unparks upon lock release. Assigned once during enquing. |
1212 |
* Upon cancellation, we do NOT adjust this field, but simply |
1213 |
* traverse through next's until we hit a non-cancelled node, |
1214 |
* (or null if at end of queue). The enqueue operation does |
1215 |
* not assign next field of a predecessor until after |
1216 |
* attachment, so seeing a null next field not necessarily |
1217 |
* mean that node is at end of queue. However, if a next field |
1218 |
* appears to be null, we can scan prev's from the tail to |
1219 |
* double-check. |
1220 |
*/ |
1221 |
transient volatile ReentrantLockQueueNode next; |
1222 |
|
1223 |
/** |
1224 |
* The thread that enqueued this node. Initialized on |
1225 |
* construction and nulled out after use. Note that this need |
1226 |
* not be declared volatile since it is always accessed after |
1227 |
* traversing volatile links, and written before writing |
1228 |
* links. |
1229 |
*/ |
1230 |
transient Thread thread; |
1231 |
|
1232 |
ReentrantLockQueueNode() { } |
1233 |
ReentrantLockQueueNode(Thread t) { thread = t; } |
1234 |
} |