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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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* @since 1.5 |
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* @spec JSR-166 |
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* @revised $Date: 2003/06/06 18:42:17 $ |
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* @editor $Author: dl $ |
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* @author Doug Lea |
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* |
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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 (i.e., volatile-write-barrier) per |
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lock/unlock pair. The "owner" field is handled as a simple |
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spinlock. To lock, the owner field is set to current thread |
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using conditional atomic update. To unlock, the owner field |
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is set to null, checking if anyone needs waking up, if so |
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doing so. Recursive locks/unlocks instead increment/decrement |
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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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algorithms. Since we must poll for cancellation of other |
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nodes, we can miss noticing whether a cancelled node is ahead |
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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 |
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queue a node is on. |
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|
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* All suspension and resumption of threads uses the internal |
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JSR166 native park/unpark API. These are safe versions of |
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suspend/resume (plus timeout support) that avoid the intrinsic |
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race problems with suspend/resume: Park suspends if not |
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preceded by an unpark. Unpark resumes if waiting, else causes |
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next park not to suspend. While safe and efficient, these are |
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not general-purpose public operations because we cannot allow |
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code outside this package to randomly call these methods -- |
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parks and unparks should be matched up. (It is OK to have more |
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unparks than unparks, but it causes threads to spuriously wake |
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up. So minimizing excessive unparks is a 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 -- we need head and tail nodes to be |
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initialized to a head node, so use the ReeantrantLock itself |
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as that node. |
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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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* 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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* Main locking code, parameterized across different policies. |
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* @param current current thread |
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* @param node its wait-node, if it already exists; else null in |
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* which case it is created, |
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* @param interruptible - true if can abort for interrupt or timeout |
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* @param nanos time to wait, or zero if untimed |
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* @return true if lock acquired (can be false only if interruptible) |
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*/ |
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final boolean doLock(Thread current, |
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ReentrantLockQueueNode node, |
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boolean interruptible, |
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long nanos) { |
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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 true; |
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} |
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|
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long lastTime = 0; // for adjusting timeouts, below |
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boolean interrupted = false; // for restoring interrupt status on exit |
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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; |
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|
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/* |
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* Repeatedly try to get ownership if first in queue, else |
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* block. |
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*/ |
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for (;;) { |
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/* |
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* 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. |
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*/ |
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|
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if (p == head && acquireOwner(current)) { |
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if (interrupted) // re-interrupt on exit |
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current.interrupt(); |
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|
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p.next = null; // clear for GC and to suppress signals |
368 |
node.thread = null; |
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node.prev = null; |
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head = node; |
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return true; |
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} |
373 |
|
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int releaseStatus = p.releaseStatus; |
375 |
|
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/* |
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* If our predecessor was cancelled, use its predecessor. |
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* There will always be a non-cancelled one somewhere |
379 |
* because head node is never cancelled, so at worst we |
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* will hit it. (Note that because head is never |
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* cancelled, we can perform this check after trying to |
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* acquire ownership). |
383 |
*/ |
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if (releaseStatus == CANCELLED) { |
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node.prev = p = p.prev; |
386 |
} |
387 |
|
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/* |
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* Wait if we are not not first in queue, or if we are |
390 |
* first, we have tried to acquire owner and failed since |
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* either entry or last release. (Note that releaseStatus can |
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* already be less than zero if we spuriously returned |
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* from a previous park or got new a predecessor due to |
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* cancellation.) |
395 |
* |
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* We also don't wait if atomic decrement of releaseStatus |
397 |
* fails. We just continue main loop on failure to |
398 |
* atomically update releaseStatus because interference |
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* causing failure is almost surely due to someone |
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* releasing us anyway. |
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* |
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* Each wait consumes all available releases. Normally |
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* there is only one anyway because release doesn't bother |
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* incrementing if already positive. |
405 |
* |
406 |
*/ |
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else if (casReleaseStatus(p, releaseStatus, |
408 |
(releaseStatus > 0)? 0 : -1) && |
409 |
releaseStatus <= 0) { |
410 |
|
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// Update and check timeout value |
412 |
if (nanos > 0) { |
413 |
long now = TimeUnit.nanoTime(); |
414 |
if (lastTime != 0) { |
415 |
nanos -= now - lastTime; |
416 |
if (nanos == 0) // avoid zero |
417 |
nanos = -1; |
418 |
} |
419 |
lastTime = now; |
420 |
} |
421 |
|
422 |
if (nanos >= 0) |
423 |
JSR166Support.park(false, nanos); |
424 |
|
425 |
if (!interruptible) { |
426 |
if (Thread.interrupted()) // consume interrupt for now |
427 |
interrupted = true; |
428 |
} |
429 |
else if (nanos < 0 || current.isInterrupted()) { |
430 |
node.thread = null; // disable signals |
431 |
releaseStatusUpdater.set(node, CANCELLED); // don't need CAS here |
432 |
signalSuccessor(node); |
433 |
return false; |
434 |
} |
435 |
} |
436 |
} |
437 |
} |
438 |
|
439 |
/** |
440 |
* Wake up node's successor, if one exists |
441 |
*/ |
442 |
private void signalSuccessor(ReentrantLockQueueNode node) { |
443 |
/* |
444 |
* Find successor -- normally just node.next. |
445 |
*/ |
446 |
ReentrantLockQueueNode s = node.next; |
447 |
|
448 |
/* |
449 |
* if s is cancelled, traverse through next's. |
450 |
*/ |
451 |
|
452 |
while (s != null && s.releaseStatus == CANCELLED) { |
453 |
node = s; |
454 |
s = s.next; |
455 |
} |
456 |
|
457 |
/* |
458 |
* If successor appears to be null, check to see if a newly |
459 |
* queued node is successor by starting at tail and working |
460 |
* backwards. If so, help out the enqueing thread by setting |
461 |
* next field. We don't expect this loop to trigger often, |
462 |
* and hardly ever to iterate. |
463 |
*/ |
464 |
|
465 |
if (s == null) { |
466 |
ReentrantLockQueueNode t = tail; |
467 |
for (;;) { |
468 |
/* |
469 |
* If t == node, there is no successor. |
470 |
*/ |
471 |
if (t == node) |
472 |
return; |
473 |
|
474 |
ReentrantLockQueueNode tp = t.prev; |
475 |
|
476 |
/* |
477 |
* t's predecessor is null if we are lagging so far |
478 |
* behind the actions of other nodes/threads that an |
479 |
* intervening head.prev was nulled by some |
480 |
* non-cancelled successor of node. In which case, |
481 |
* there's no live successor. |
482 |
*/ |
483 |
|
484 |
if (tp == null) |
485 |
return; |
486 |
|
487 |
/* |
488 |
* If we find successor, we can do the assignment to |
489 |
* next (don't even need CAS) on behalf of enqueuing |
490 |
* thread. At worst we will stall now and lag behind |
491 |
* both the setting and the later clearing of next |
492 |
* field. But if so, we will reattach an internal link |
493 |
* in soon-to-be unreachable set of nodes, so no harm |
494 |
* done. |
495 |
*/ |
496 |
|
497 |
if (tp == node) { |
498 |
node.next = s = t; |
499 |
break; |
500 |
} |
501 |
|
502 |
t = tp; |
503 |
|
504 |
/* |
505 |
* Before iterating, check to see if link has |
506 |
* appeared. |
507 |
*/ |
508 |
ReentrantLockQueueNode n = node.next; |
509 |
if (n != null) { |
510 |
s = n; |
511 |
break; |
512 |
} |
513 |
} |
514 |
} |
515 |
|
516 |
Thread thr = s.thread; |
517 |
if (thr != null && thr != owner) // don't bother signalling if has lock |
518 |
JSR166Support.unpark(thr); |
519 |
} |
520 |
|
521 |
|
522 |
/** |
523 |
* Increment releaseStatus and signal next thread in queue if one |
524 |
* exists and is waiting. Called only by unlock. This code is split |
525 |
* out from unlock to encourage inlining of non-contended cases. |
526 |
*/ |
527 |
private void releaseFirst() { |
528 |
for (;;) { |
529 |
ReentrantLockQueueNode h = head; |
530 |
if (h == tail) // No successor |
531 |
return; |
532 |
|
533 |
int c = h.releaseStatus; |
534 |
if (c > 0) // Don't need signal if already positive |
535 |
return; |
536 |
if (owner != null) // Don't bother if some thread got lock |
537 |
return; |
538 |
|
539 |
if (casReleaseStatus(h, c, (c < 0)? 0 : 1)) { // saturate at 1 |
540 |
if (c < 0) |
541 |
signalSuccessor(h); |
542 |
return; |
543 |
} |
544 |
// else retry if CAS fails |
545 |
} |
546 |
} |
547 |
|
548 |
/** |
549 |
* Attempts to release this lock. <p>If the current thread is the |
550 |
* holder of this lock then the hold count is decremented. If the |
551 |
* hold count is now zero then the lock is released. <p>If the |
552 |
* current thread is not the holder of this lock then {@link |
553 |
* IllegalMonitorStateException} is thrown. |
554 |
* @throws IllegalMonitorStateException if the current thread does not |
555 |
* hold this lock. |
556 |
*/ |
557 |
public void unlock() { |
558 |
checkOwner(Thread.currentThread()); |
559 |
if (recursions > 0) |
560 |
--recursions; |
561 |
else { |
562 |
ownerUpdater.set(this, null); |
563 |
if (tail != this) // don't bother if never contended |
564 |
releaseFirst(); |
565 |
} |
566 |
} |
567 |
|
568 |
/** |
569 |
* Acquire the lock. |
570 |
* <p>Acquires the lock if it is not held be another thread and returns |
571 |
* immediately, setting the lock hold count to one. |
572 |
* <p>If the current thread |
573 |
* already holds the lock then the hold count is incremented by one and |
574 |
* the method returns immediately. |
575 |
* <p>If the lock is held by another thread then the |
576 |
* the current thread thread becomes disabled for thread scheduling |
577 |
* purposes and lies dormant until the lock has been acquired |
578 |
* at which time the lock hold count is set to one. |
579 |
*/ |
580 |
public void lock() { |
581 |
Thread current = Thread.currentThread(); |
582 |
if (!canBarge() || !acquireOwner(current)) |
583 |
doLock(current, null, false, 0); |
584 |
|
585 |
} |
586 |
|
587 |
/** |
588 |
* Acquires the lock unless the current thread is |
589 |
* {@link Thread#interrupt interrupted}. |
590 |
* <p>Acquires the lock if it is not held by another thread and returns |
591 |
* immediately, setting the lock hold count to one. |
592 |
* <p>If the current thread already holds this lock then the hold count |
593 |
* is incremented by one and the method returns immediately. |
594 |
* <p>If the lock is held by another thread then the |
595 |
* the current thread becomes disabled for thread scheduling |
596 |
* purposes and lies dormant until one of two things happens: |
597 |
* <ul> |
598 |
* <li> The lock is acquired by the current thread; or |
599 |
* <li> Some other thread {@link Thread#interrupt interrupts} the current |
600 |
* thread. |
601 |
* </ul> |
602 |
* <p>If the lock is acquired by the current thread then the lock hold |
603 |
* count is set to one. |
604 |
* <p>If the current thread: |
605 |
* <ul> |
606 |
* <li>has its interrupted status set on entry to this method; or |
607 |
* <li>is {@link Thread#interrupt interrupted} while waiting to acquire |
608 |
* the lock, |
609 |
* </ul> |
610 |
* then {@link InterruptedException} is thrown and the current thread's |
611 |
* interrupted status is cleared. As this method is an explicit |
612 |
* interruption point, preference is given to responding to the interrupt |
613 |
* over reentrant acquisition of the lock. |
614 |
* |
615 |
* |
616 |
* @throws InterruptedException if the current thread is interrupted |
617 |
*/ |
618 |
public void lockInterruptibly() throws InterruptedException { |
619 |
Thread current = Thread.currentThread(); |
620 |
if (Thread.interrupted() || |
621 |
(!canBarge() || !acquireOwner(current)) && |
622 |
!doLock(current, null, true, 0)) |
623 |
throw new InterruptedException(); |
624 |
} |
625 |
|
626 |
/** |
627 |
* Acquires the lock only if it is not held by another thread at the time |
628 |
* of invocation. |
629 |
* <p>Acquires the lock if it is not held by another thread and returns |
630 |
* immediately with the value <tt>true</tt>, setting the lock hold count |
631 |
* to one. |
632 |
* <p> If the current thread |
633 |
* already holds this lock then the hold count is incremented by one and |
634 |
* the method returns <tt>true</tt>. |
635 |
* <p>If the lock is held by another thread then this method will return |
636 |
* immediately with the value <tt>false</tt>. |
637 |
* |
638 |
* @return <tt>true</tt>if the lock was free and was acquired by the |
639 |
* current thread, or the lock was already held by the current thread; and |
640 |
* <tt>false</tt> otherwise. |
641 |
*/ |
642 |
public boolean tryLock() { |
643 |
Thread current = Thread.currentThread(); |
644 |
if (acquireOwner(current)) |
645 |
return true; |
646 |
if (owner == current) { |
647 |
++recursions; |
648 |
return true; |
649 |
} |
650 |
return false; |
651 |
} |
652 |
|
653 |
/** |
654 |
* |
655 |
* Acquires the lock if it is not held by another thread within the given |
656 |
* waiting time and the current thread has not been interrupted. |
657 |
* <p>Acquires the lock if it is not held by another thread and returns |
658 |
* immediately with the value <tt>true</tt>, setting the lock hold count |
659 |
* to one. |
660 |
* <p> If the current thread |
661 |
* already holds this lock then the hold count is incremented by one and |
662 |
* the method returns <tt>true</tt>. |
663 |
* <p>If the lock is held by another thread then the |
664 |
* the current thread becomes disabled for thread scheduling |
665 |
* purposes and lies dormant until one of three things happens: |
666 |
* <ul> |
667 |
* <li> The lock is acquired by the current thread; or |
668 |
* <li> Some other thread {@link Thread#interrupt interrupts} the current |
669 |
* thread; or |
670 |
* <li> The specified waiting time elapses |
671 |
* </ul> |
672 |
* <p>If the lock is acquired then the value <tt>true</tt> is returned and |
673 |
* the lock hold count is set to one. |
674 |
* <p>If the current thread: |
675 |
* <ul> |
676 |
* <li>has its interrupted status set on entry to this method; or |
677 |
* <li>is {@link Thread#interrupt interrupted} before acquiring |
678 |
* the lock, |
679 |
* </ul> |
680 |
* then {@link InterruptedException} is thrown and the current thread's |
681 |
* interrupted status is cleared. As this method is an explicit |
682 |
* interruption point, preference is given to responding to the interrupt |
683 |
* over reentrant acquisition of the lock. |
684 |
* <p>If the specified waiting time elapses then the value <tt>false</tt> |
685 |
* is returned. |
686 |
* <p>The given waiting time is a best-effort lower bound. If the time is |
687 |
* less than or equal to zero, the method will not wait at all. |
688 |
* |
689 |
* |
690 |
* @return <tt>true</tt> if the lock was free and was acquired by the |
691 |
* current thread, or the lock was already held by the current thread; and |
692 |
* <tt>false</tt> if the waiting time elapsed before the lock could be |
693 |
* acquired. |
694 |
* |
695 |
* @throws InterruptedException if the current thread is interrupted |
696 |
*/ |
697 |
public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { |
698 |
if (unit == null) |
699 |
throw new NullPointerException(); |
700 |
if (Thread.interrupted()) |
701 |
throw new InterruptedException(); |
702 |
Thread current = Thread.currentThread(); |
703 |
if (canBarge() && acquireOwner(current)) |
704 |
return true; |
705 |
if (owner == current) { // check recursions before timeout |
706 |
++recursions; |
707 |
return true; |
708 |
} |
709 |
if (timeout <= 0) |
710 |
return false; |
711 |
if (doLock(current, null, true, unit.toNanos(timeout))) |
712 |
return true; |
713 |
if (Thread.interrupted()) |
714 |
throw new InterruptedException(); |
715 |
return false; // timed out |
716 |
} |
717 |
|
718 |
|
719 |
/** |
720 |
* Queries the number of holds on this lock by the current thread. |
721 |
* <p>A thread has a hold on a lock for each lock action that is not |
722 |
* matched by an unlock action. |
723 |
* <p>The hold count information is typically only used for testing and |
724 |
* debugging purposes. For example, if a certain section of code should |
725 |
* not be entered with the lock already held then we can assert that |
726 |
* fact: |
727 |
* <pre> |
728 |
* class X { |
729 |
* ReentrantLock lock = new ReentrantLock(); |
730 |
* // ... |
731 |
* |
732 |
* public void m() { |
733 |
* assert lock.getHoldCount() == 0; |
734 |
* lock.lock(); |
735 |
* try { |
736 |
* // ... method body |
737 |
* } finally { |
738 |
* lock.unlock() |
739 |
* } |
740 |
* } |
741 |
* } |
742 |
* </pre> |
743 |
* |
744 |
* @return the number of holds on this lock by current thread, |
745 |
* or zero if this lock is not held by current thread. |
746 |
**/ |
747 |
public int getHoldCount() { |
748 |
return (owner == Thread.currentThread()) ? recursions + 1 : 0; |
749 |
} |
750 |
|
751 |
/** |
752 |
* Queries if this lock is held by the current thread. |
753 |
* <p>Analogous to the {@link Thread#holdsLock} method for built-in |
754 |
* monitor locks, this method is typically used for debugging and |
755 |
* testing. For example, a method that should only be called while |
756 |
* a lock is held can assert that this is the case: |
757 |
* <pre> |
758 |
* class X { |
759 |
* ReentrantLock lock = new ReentrantLock(); |
760 |
* // ... |
761 |
* |
762 |
* public void m() { |
763 |
* assert lock.isHeldByCurrentThread(); |
764 |
* // ... method body |
765 |
* } |
766 |
* } |
767 |
* </pre> |
768 |
* |
769 |
* @return <tt>true</tt> if current thread holds this lock and |
770 |
* <tt>false</tt> otherwise. |
771 |
**/ |
772 |
public boolean isHeldByCurrentThread() { |
773 |
return (owner == Thread.currentThread()); |
774 |
} |
775 |
|
776 |
|
777 |
/** |
778 |
* Queries if this lock is held by any thread. THis method is |
779 |
* designed for use in monitoring, not for synchronization control. |
780 |
* @return <tt>true</tt> if any thread holds this lock and |
781 |
* <tt>false</tt> otherwise. |
782 |
**/ |
783 |
public boolean isLocked() { |
784 |
return owner != null; |
785 |
} |
786 |
|
787 |
/** |
788 |
* Reconstitute by resetting head and tail to point back to the lock. |
789 |
*/ |
790 |
private void readObject(java.io.ObjectInputStream s) |
791 |
throws java.io.IOException, ClassNotFoundException { |
792 |
s.defaultReadObject(); |
793 |
head = tail = this; |
794 |
} |
795 |
|
796 |
/** |
797 |
* Returns a {@link Condition} instance for use with this |
798 |
* {@link Lock} instance. |
799 |
* |
800 |
* <p>The returned {@link Condition} instance has the same behavior and |
801 |
* usage |
802 |
* restrictions with this lock as the {@link Object} monitor methods |
803 |
* ({@link Object#wait() wait}, {@link Object#notify notify}, and |
804 |
* {@link Object#notifyAll notifyAll}) have with the built-in monitor |
805 |
* lock: |
806 |
* <ul> |
807 |
* <li>If this lock is not held when any of the {@link Condition} |
808 |
* {@link Condition#await() waiting} or {@link Condition#signal signalling} |
809 |
* methods are called, then an {@link IllegalMonitorStateException} is |
810 |
* thrown. |
811 |
* <li>When the condition {@link Condition#await() waiting} methods are |
812 |
* called the lock is released and before they return the lock is |
813 |
* reacquired and the lock hold count restored to what it was when the |
814 |
* method was called. |
815 |
* <li>If a thread is {@link Thread#interrupt interrupted} while waiting |
816 |
* then the wait will terminate, an {@link InterruptedException} will be |
817 |
* thrown, and the thread's interrupted status will be cleared. |
818 |
* <li>The order in which waiting threads are signalled is not specified. |
819 |
* <li>The order in which threads returning from a wait, and threads trying |
820 |
* to acquire the lock, are granted the lock, is not specified. |
821 |
* </ul> |
822 |
*/ |
823 |
public Condition newCondition() { |
824 |
return new ReentrantLockConditionObject(); |
825 |
} |
826 |
|
827 |
// Helper methods for Conditions |
828 |
|
829 |
/** |
830 |
* Return true if a node, always one that was initially placed on |
831 |
* a condition queue, is off the condition queue (and thus, |
832 |
* normally is now on lock queue.) |
833 |
*/ |
834 |
boolean isOffConditionQueue(ReentrantLockQueueNode w) { |
835 |
return w.releaseStatus > TRANSFERRING; |
836 |
} |
837 |
|
838 |
/** |
839 |
* Transfer a node from a condition queue onto lock queue. |
840 |
* Return true if successful (i.e., node not cancelled) |
841 |
*/ |
842 |
final boolean transferToLockQueue(ReentrantLockQueueNode node) { |
843 |
/* |
844 |
* Atomically change status to TRANSFERRING to avoid races |
845 |
* with cancelling waiters. We use a special value that causes |
846 |
* any waiters spuriously waking up to re-park until the node |
847 |
* has been placed on lock queue. |
848 |
*/ |
849 |
if (!casReleaseStatus(node, ON_CONDITION_QUEUE, TRANSFERRING)) |
850 |
return false; |
851 |
|
852 |
/* |
853 |
* Splice onto queue |
854 |
*/ |
855 |
ReentrantLockQueueNode p = enq(node); |
856 |
|
857 |
/* |
858 |
* Establish normal lock-queue releaseStatus for node. The |
859 |
* CAS can fail if node already was involved in a cancellation |
860 |
* on lock-queue, in which case we signal to be sure. |
861 |
*/ |
862 |
if (!casReleaseStatus(node, TRANSFERRING, 0)) |
863 |
signalSuccessor(node); |
864 |
|
865 |
/* |
866 |
* Ensure releaseStatus of predecessor is negative to indicate |
867 |
* that thread is (probably) waiting. If attempt to set releaseStatus |
868 |
* fails or is pred is/becomes cancelled, wake up successor |
869 |
* (which will ordinarily be "node") to resynch. |
870 |
*/ |
871 |
|
872 |
for (;;) { |
873 |
int c = p.releaseStatus; |
874 |
if (c < 0 || (c != CANCELLED && casReleaseStatus(p, c, -1))) |
875 |
break; |
876 |
signalSuccessor(p); |
877 |
if (c == CANCELLED) |
878 |
break; |
879 |
} |
880 |
|
881 |
return true; |
882 |
} |
883 |
|
884 |
/** |
885 |
* Hook method used by ReentrantReadWriteLock. Called |
886 |
* before unlocking lock to enter wait. |
887 |
*/ |
888 |
void beforeWait() { } |
889 |
|
890 |
|
891 |
/** |
892 |
* Hook method used by ReentrantReadWriteLock. Called |
893 |
* after locking lock after exiting wait. |
894 |
*/ |
895 |
void afterWait() { } |
896 |
|
897 |
private class ReentrantLockConditionObject implements Condition, java.io.Serializable { |
898 |
/* |
899 |
* Because condition queues are accessed only when locks are |
900 |
* already held, we just need a simple linked queue to hold |
901 |
* nodes while they are waiting on conditions. They are then |
902 |
* transferred to the lock queue to re-acquire locks. |
903 |
*/ |
904 |
|
905 |
/** |
906 |
* First node of condition queue. |
907 |
*/ |
908 |
private transient ReentrantLockQueueNode firstWaiter; |
909 |
|
910 |
/** |
911 |
* Last node of condition queue. |
912 |
*/ |
913 |
private transient ReentrantLockQueueNode lastWaiter; |
914 |
|
915 |
/** |
916 |
* Basic linked queue insertion. |
917 |
*/ |
918 |
private ReentrantLockQueueNode addWaiter(Thread current) { |
919 |
ReentrantLockQueueNode w = new ReentrantLockQueueNode(current); |
920 |
w.releaseStatus = ON_CONDITION_QUEUE; |
921 |
if (lastWaiter == null) |
922 |
firstWaiter = lastWaiter = w; |
923 |
else { |
924 |
ReentrantLockQueueNode t = lastWaiter; |
925 |
lastWaiter = w; |
926 |
t.next = w; |
927 |
} |
928 |
return w; |
929 |
} |
930 |
|
931 |
/** |
932 |
* Main code for signal. Dequeue and transfer nodes until hit |
933 |
* non-cancelled one or null. Split out from signal to |
934 |
* encourage compilers to inline the case of no waiters. |
935 |
*/ |
936 |
private void doSignal(ReentrantLockQueueNode first) { |
937 |
do { |
938 |
if ( (firstWaiter = first.next) == null) |
939 |
lastWaiter = null; |
940 |
first.next = null; |
941 |
if (transferToLockQueue(first)) |
942 |
return; |
943 |
first = firstWaiter; |
944 |
} while (first != null); |
945 |
} |
946 |
|
947 |
public void signal() { |
948 |
checkOwner(Thread.currentThread()); |
949 |
ReentrantLockQueueNode w = firstWaiter; |
950 |
if (w != null) |
951 |
doSignal(w); |
952 |
} |
953 |
|
954 |
public void signalAll() { |
955 |
checkOwner(Thread.currentThread()); |
956 |
// Pull off list all at once and traverse. |
957 |
ReentrantLockQueueNode w = firstWaiter; |
958 |
if (w != null) { |
959 |
lastWaiter = firstWaiter = null; |
960 |
do { |
961 |
ReentrantLockQueueNode n = w.next; |
962 |
w.next = null; |
963 |
transferToLockQueue(w); |
964 |
w = n; |
965 |
} while (w != null); |
966 |
} |
967 |
} |
968 |
|
969 |
/* |
970 |
* Various flavors of wait. Each almost the same, but |
971 |
* annoyingly different and no nice way to factor common code. |
972 |
*/ |
973 |
|
974 |
public void await() throws InterruptedException { |
975 |
Thread current = Thread.currentThread(); |
976 |
checkOwner(current); |
977 |
|
978 |
ReentrantLockQueueNode w = addWaiter(current); |
979 |
beforeWait(); |
980 |
int recs = recursions; |
981 |
unlock(); |
982 |
|
983 |
boolean wasInterrupted = false; |
984 |
|
985 |
while (!isOffConditionQueue(w)) { |
986 |
JSR166Support.park(false, 0); |
987 |
if (Thread.interrupted()) { |
988 |
wasInterrupted = true; |
989 |
if (casReleaseStatus(w, ON_CONDITION_QUEUE, CANCELLED)) { |
990 |
w.thread = null; |
991 |
w = null; |
992 |
} |
993 |
break; |
994 |
} |
995 |
} |
996 |
|
997 |
/* |
998 |
* If we exited above loop due to cancellation, then w is |
999 |
* null, and doLock will make a new lock node for |
1000 |
* us. Otherwise, upon exit, our node is already in the |
1001 |
* lock queue when doLock is called. |
1002 |
*/ |
1003 |
doLock(current, w, false, 0); |
1004 |
|
1005 |
recursions = recs; |
1006 |
afterWait(); |
1007 |
|
1008 |
if (wasInterrupted || Thread.interrupted()) |
1009 |
throw new InterruptedException(); |
1010 |
} |
1011 |
|
1012 |
public void awaitUninterruptibly() { |
1013 |
Thread current = Thread.currentThread(); |
1014 |
checkOwner(current); |
1015 |
|
1016 |
ReentrantLockQueueNode w = addWaiter(current); |
1017 |
beforeWait(); |
1018 |
int recs = recursions; |
1019 |
unlock(); |
1020 |
|
1021 |
boolean wasInterrupted = false; |
1022 |
while (!isOffConditionQueue(w)) { |
1023 |
JSR166Support.park(false, 0); |
1024 |
if (Thread.interrupted()) |
1025 |
wasInterrupted = true; |
1026 |
} |
1027 |
|
1028 |
doLock(current, w, false, 0); |
1029 |
recursions = recs; |
1030 |
afterWait(); |
1031 |
// avoid re-interrupts on exit |
1032 |
if (wasInterrupted && !current.isInterrupted()) |
1033 |
current.interrupt(); |
1034 |
} |
1035 |
|
1036 |
|
1037 |
public long awaitNanos(long nanos) throws InterruptedException { |
1038 |
Thread current = Thread.currentThread(); |
1039 |
checkOwner(current); |
1040 |
|
1041 |
ReentrantLockQueueNode w = addWaiter(current); |
1042 |
beforeWait(); |
1043 |
int recs = recursions; |
1044 |
unlock(); |
1045 |
|
1046 |
if (nanos <= 0) nanos = 1; // park arg must be positive |
1047 |
long timeLeft = nanos; |
1048 |
long startTime = TimeUnit.nanoTime(); |
1049 |
boolean wasInterrupted = false; |
1050 |
boolean cancelled = false; |
1051 |
|
1052 |
if (!isOffConditionQueue(w)) { |
1053 |
for (;;) { |
1054 |
JSR166Support.park(false, timeLeft); |
1055 |
if (Thread.interrupted()) |
1056 |
wasInterrupted = true; |
1057 |
else if (isOffConditionQueue(w)) |
1058 |
break; |
1059 |
else |
1060 |
timeLeft = nanos - (TimeUnit.nanoTime() - startTime); |
1061 |
|
1062 |
if (wasInterrupted || timeLeft <= 0) { |
1063 |
if (casReleaseStatus(w, ON_CONDITION_QUEUE, CANCELLED)) { |
1064 |
w.thread = null; |
1065 |
w = null; |
1066 |
} |
1067 |
break; |
1068 |
} |
1069 |
} |
1070 |
} |
1071 |
|
1072 |
doLock(current, w, false, 0); |
1073 |
recursions = recs; |
1074 |
afterWait(); |
1075 |
|
1076 |
if (wasInterrupted || Thread.interrupted()) |
1077 |
throw new InterruptedException(); |
1078 |
else if (timeLeft <= 0) |
1079 |
return timeLeft; |
1080 |
else |
1081 |
return nanos - (TimeUnit.nanoTime() - startTime); |
1082 |
} |
1083 |
|
1084 |
public boolean awaitUntil(Date deadline) throws InterruptedException { |
1085 |
Thread current = Thread.currentThread(); |
1086 |
checkOwner(current); |
1087 |
|
1088 |
ReentrantLockQueueNode w = addWaiter(current); |
1089 |
beforeWait(); |
1090 |
int recs = recursions; |
1091 |
unlock(); |
1092 |
|
1093 |
boolean wasInterrupted = false; |
1094 |
boolean cancelled = false; |
1095 |
long abstime = deadline.getTime(); |
1096 |
|
1097 |
if (!isOffConditionQueue(w)) { |
1098 |
for (;;) { |
1099 |
JSR166Support.park(true, abstime); |
1100 |
|
1101 |
boolean timedOut = false; |
1102 |
if (Thread.interrupted()) |
1103 |
wasInterrupted = true; |
1104 |
else if (isOffConditionQueue(w)) |
1105 |
break; |
1106 |
else if (System.currentTimeMillis() <= abstime) |
1107 |
timedOut = true; |
1108 |
|
1109 |
if (wasInterrupted || timedOut) { |
1110 |
if (casReleaseStatus(w, ON_CONDITION_QUEUE, CANCELLED)) { |
1111 |
w.thread = null; |
1112 |
w = null; |
1113 |
} |
1114 |
break; |
1115 |
} |
1116 |
} |
1117 |
} |
1118 |
|
1119 |
doLock(current, w, false, 0); |
1120 |
recursions = recs; |
1121 |
afterWait(); |
1122 |
|
1123 |
if (wasInterrupted || Thread.interrupted()) |
1124 |
throw new InterruptedException(); |
1125 |
return !cancelled; |
1126 |
} |
1127 |
|
1128 |
public boolean await(long time, TimeUnit unit) throws InterruptedException { |
1129 |
return awaitNanos(unit.toNanos(time)) > 0; |
1130 |
} |
1131 |
|
1132 |
} |
1133 |
|
1134 |
} |
1135 |
|
1136 |
/** |
1137 |
* Node class for threads waiting for locks. This cannot be nested |
1138 |
* inside ReentrantLock because of Java inheritance circularity rules. |
1139 |
*/ |
1140 |
class ReentrantLockQueueNode { |
1141 |
/** |
1142 |
* Controls whether successor node is allowed to try to obtain |
1143 |
* ownership. Acts as a saturating (in both directions) counting |
1144 |
* semaphore: Upon each wait, the releaseStatus is reduced to zero |
1145 |
* if positive, else reduced to negative, in which case the thread |
1146 |
* will park. The releaseStatus is incremented on each unlock that |
1147 |
* would enable successor thread to obtain lock (succeeding if |
1148 |
* there is no contention). The special value of CANCELLED is used |
1149 |
* to mean that the releaseStatus cannot be either incremented or |
1150 |
* decremented. The special value of ON_CONDITION_QUEUE is used |
1151 |
* when nodes are on conditions queues instead of lock queue, and |
1152 |
* the special value TRANSFERRING is used while signals are in |
1153 |
* progress. |
1154 |
*/ |
1155 |
transient volatile int releaseStatus; |
1156 |
|
1157 |
/** |
1158 |
* Link to predecessor node that current node/thread relies on |
1159 |
* for checking releaseStatus. Assigned once during enqueing, |
1160 |
* and nulled out (for sake of GC) only upon dequeuing. Upon |
1161 |
* cancellation, we do NOT adjust this field, but simply |
1162 |
* traverse through prev's until we hit a non-cancelled node. |
1163 |
* A valid predecessor will always exist because the head node |
1164 |
* is never cancelled. |
1165 |
*/ |
1166 |
transient volatile ReentrantLockQueueNode prev; |
1167 |
|
1168 |
/** |
1169 |
* Link to the successor node that the current node/thread |
1170 |
* unparks upon lock release. Assigned once during enquing. |
1171 |
* Upon cancellation, we do NOT adjust this field, but simply |
1172 |
* traverse through next's until we hit a non-cancelled node, |
1173 |
* (or null if at end of queue). The enqueue operation does |
1174 |
* not assign next field of a predecessor until after |
1175 |
* attachment, so seeing a null next field not necessarily |
1176 |
* mean that node is at end of queue. However, if a next field |
1177 |
* appears to be null, we can scan prev's from the tail to |
1178 |
* double-check. |
1179 |
*/ |
1180 |
transient volatile ReentrantLockQueueNode next; |
1181 |
|
1182 |
/** |
1183 |
* The thread that enqueued this node. Initialized on |
1184 |
* construction and nulled out after use. Note that this need |
1185 |
* not be declared volatile since it is always accessed after |
1186 |
* traversing volatile links, and written before writing |
1187 |
* links. |
1188 |
*/ |
1189 |
transient Thread thread; |
1190 |
|
1191 |
ReentrantLockQueueNode() { } |
1192 |
ReentrantLockQueueNode(Thread t) { thread = t; } |
1193 |
} |