8301341: LinkedTransferQueue does not respect timeout for poll()
8300663: java/util/concurrent/SynchronousQueue/Fairness.java failed with "Error: fair=true i=0 j=1" 8267502: JDK-8246677 caused 16x performance regression in SynchronousQueue Reviewed-by: alanb
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Doug Lea
parent
bfa76dffb5
commit
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@ -47,6 +47,9 @@ import java.util.Spliterator;
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import java.util.Spliterators;
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import java.util.concurrent.locks.LockSupport;
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import java.util.concurrent.locks.ReentrantLock;
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import java.util.concurrent.ForkJoinWorkerThread;
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import java.util.concurrent.LinkedTransferQueue;
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import java.util.concurrent.TransferQueue;
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/**
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* A {@linkplain BlockingQueue blocking queue} in which each insert
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@ -98,717 +101,137 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
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* M. L. Scott. 18th Annual Conf. on Distributed Computing,
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* Oct. 2004 (see also
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* http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).
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* The (Lifo) stack is used for non-fair mode, and the (Fifo)
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* queue for fair mode. The performance of the two is generally
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* similar. Fifo usually supports higher throughput under
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* contention but Lifo maintains higher thread locality in common
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* applications.
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* The queue is treated as a Lifo stack in non-fair mode, and a
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* Fifo queue in fair mode. In most contexts, transfer performance
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* is roughly comparable across them. Lifo is usually faster under
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* low contention, but slower under high contention. Performance
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* of applications using them also varies. Lifo is generally
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* preferable in resource management settings (for example cached
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* thread pools) because of better temporal locality, but
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* inappropriate for message-passing applications.
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*
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* A dual queue (and similarly stack) is one that at any given
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* time either holds "data" -- items provided by put operations,
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* or "requests" -- slots representing take operations, or is
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* empty. A call to "fulfill" (i.e., a call requesting an item
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* from a queue holding data or vice versa) dequeues a
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* complementary node. The most interesting feature of these
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* queues is that any operation can figure out which mode the
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* queue is in, and act accordingly without needing locks.
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*
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* Both the queue and stack extend abstract class Transferer
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* defining the single method transfer that does a put or a
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* take. These are unified into a single method because in dual
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* data structures, the put and take operations are symmetrical,
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* so nearly all code can be combined. The resulting transfer
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* methods are on the long side, but are easier to follow than
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* they would be if broken up into nearly-duplicated parts.
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*
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* The queue and stack data structures share many conceptual
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* similarities but very few concrete details. For simplicity,
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* they are kept distinct so that they can later evolve
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* separately.
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* A dual queue is one that at any given time either holds "data"
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* -- items provided by put operations, or "requests" -- slots
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* representing take operations, or is empty. A fulfilling
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* operation (i.e., a call requesting an item from a queue holding
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* data or vice versa) "matches" the item of and then dequeues a
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* complementary node. Any operation can figure out which mode
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* the queue is in, and act accordingly without needing locks. So
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* put and take operations are symmetrical, and all transfer
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* methods invoke a single "xfer" method that does a put or a take
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* in either fifo or lifo mode.
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*
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* The algorithms here differ from the versions in the above paper
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* in extending them for use in synchronous queues, as well as
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* dealing with cancellation. The main differences include:
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* in ways including:
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*
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* 1. The original algorithms used bit-marked pointers, but
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* the ones here use mode bits in nodes, leading to a number
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* of further adaptations.
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* 2. SynchronousQueues must block threads waiting to become
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* fulfilled.
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* 3. Support for cancellation via timeout and interrupts,
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* including cleaning out cancelled nodes/threads
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* from lists to avoid garbage retention and memory depletion.
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*
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* Blocking is mainly accomplished using LockSupport park/unpark,
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* except that nodes that appear to be the next ones to become
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* fulfilled first spin a bit (on multiprocessors only). On very
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* busy synchronous queues, spinning can dramatically improve
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* throughput. And on less busy ones, the amount of spinning is
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* small enough not to be noticeable.
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*
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* Cleaning is done in different ways in queues vs stacks. For
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* queues, we can almost always remove a node immediately in O(1)
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* time (modulo retries for consistency checks) when it is
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* cancelled. But if it may be pinned as the current tail, it must
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* wait until some subsequent cancellation. For stacks, we need a
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* potentially O(n) traversal to be sure that we can remove the
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* node, but this can run concurrently with other threads
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* accessing the stack.
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*
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* While garbage collection takes care of most node reclamation
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* issues that otherwise complicate nonblocking algorithms, care
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* is taken to "forget" references to data, other nodes, and
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* threads that might be held on to long-term by blocked
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* threads. In cases where setting to null would otherwise
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* conflict with main algorithms, this is done by changing a
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* node's link to now point to the node itself. This doesn't arise
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* much for Stack nodes (because blocked threads do not hang on to
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* old head pointers), but references in Queue nodes must be
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* aggressively forgotten to avoid reachability of everything any
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* node has ever referred to since arrival.
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*
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* The above steps improve throughput when many threads produce
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* and/or consume data. But they don't help much with
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* single-source / single-sink usages in which one side or the
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* other is always transiently blocked, and so throughput is
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* mainly a function of thread scheduling. This is not usually
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* noticeably improved with bounded short spin-waits. Instead both
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* forms of transfer try Thread.yield if apparently the sole
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* waiter. This works well when there are more tasks that cores,
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* which is expected to be the main usage context of this mode. In
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* other cases, waiters may help with some bookkeeping, then
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* park/unpark.
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* * The original algorithms used bit-marked pointers, but the
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* ones here use a bit (isData) in nodes, and usually avoid
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* creating nodes when fulfilling. They also use the
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* compareAndExchange form of CAS for pointer updates to
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* reduce memory traffic.
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* * Fifo mode is based on LinkedTransferQueue operations, but
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* Lifo mode support is added in subclass Transferer.
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* * The Fifo version accommodates lazy updates and slack as
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* described in LinkedTransferQueue internal documentation.
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* * Threads may block when waiting to become fulfilled,
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* sometimes preceded by brief spins.
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* * Support for cancellation via timeout and interrupts,
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* including cleaning out cancelled nodes/threads from lists
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* to avoid garbage retention and memory depletion.
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*/
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/**
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* Shared internal API for dual stacks and queues.
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* Extension of LinkedTransferQueue to support Lifo (stack) mode.
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* Methods use the "head" field as head (top) of stack (versus
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* queue). Note that popped nodes are not self-linked because they
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* are not prone to unbounded garbage chains. Also note that
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* "async" mode is never used and not supported for synchronous
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* transfers.
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*/
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abstract static class Transferer<E> {
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@SuppressWarnings("serial") // never serialized
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static final class Transferer<E> extends LinkedTransferQueue<E> {
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/**
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* Performs a put or take.
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* Puts or takes an item with lifo ordering. Loops trying:
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* * If top (var p) exists and is already matched, pop and continue
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* * If top has complementary type, try to fulfill by CASing item,
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* On success pop (which will succeed unless already helped),
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* otherwise restart.
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* * If no possible match, unless immediate mode, push a
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* node and wait, later unsplicing if cancelled.
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*
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* @param e if non-null, the item to be handed to a consumer;
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* if null, requests that transfer return an item
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* offered by producer.
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* @param timed if this operation should timeout
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* @param nanos the timeout, in nanoseconds
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* @return if non-null, the item provided or received; if null,
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* the operation failed due to timeout or interrupt --
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* the caller can distinguish which of these occurred
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* by checking Thread.interrupted.
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* @param e the item or null for take
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* @param ns timeout or 0 if immediate, Long.MAX_VALUE if untimed
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* @return an item if matched, else e
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*/
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abstract E transfer(E e, boolean timed, long nanos);
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}
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/**
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* The number of nanoseconds for which it is faster to spin
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* rather than to use timed park. A rough estimate suffices.
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*/
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static final long SPIN_FOR_TIMEOUT_THRESHOLD = 1023L;
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/** Dual stack */
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static final class TransferStack<E> extends Transferer<E> {
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/*
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* This extends Scherer-Scott dual stack algorithm, differing,
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* among other ways, by using "covering" nodes rather than
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* bit-marked pointers: Fulfilling operations push on marker
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* nodes (with FULFILLING bit set in mode) to reserve a spot
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* to match a waiting node.
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*/
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/* Modes for SNodes, ORed together in node fields */
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/** Node represents an unfulfilled consumer */
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static final int REQUEST = 0;
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/** Node represents an unfulfilled producer */
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static final int DATA = 1;
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/** Node is fulfilling another unfulfilled DATA or REQUEST */
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static final int FULFILLING = 2;
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/** Returns true if m has fulfilling bit set. */
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static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }
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/** Node class for TransferStacks. */
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static final class SNode implements ForkJoinPool.ManagedBlocker {
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volatile SNode next; // next node in stack
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volatile SNode match; // the node matched to this
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volatile Thread waiter; // to control park/unpark
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Object item; // data; or null for REQUESTs
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int mode;
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// Note: item and mode fields don't need to be volatile
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// since they are always written before, and read after,
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// other volatile/atomic operations.
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SNode(Object item) {
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this.item = item;
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}
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boolean casNext(SNode cmp, SNode val) {
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return cmp == next &&
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SNEXT.compareAndSet(this, cmp, val);
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}
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/**
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* Tries to match node s to this node, if so, waking up thread.
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* Fulfillers call tryMatch to identify their waiters.
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* Waiters block until they have been matched.
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*
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* @param s the node to match
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* @return true if successfully matched to s
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*/
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boolean tryMatch(SNode s) {
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SNode m; Thread w;
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if ((m = match) == null) {
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if (SMATCH.compareAndSet(this, null, s)) {
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if ((w = waiter) != null)
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LockSupport.unpark(w);
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return true;
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}
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else
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m = match;
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}
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return m == s;
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}
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/**
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* Tries to cancel a wait by matching node to itself.
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*/
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boolean tryCancel() {
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return SMATCH.compareAndSet(this, null, this);
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}
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boolean isCancelled() {
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return match == this;
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}
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public final boolean isReleasable() {
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return match != null || Thread.currentThread().isInterrupted();
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}
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public final boolean block() {
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while (!isReleasable()) LockSupport.park();
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return true;
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}
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void forgetWaiter() {
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SWAITER.setOpaque(this, null);
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}
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// VarHandle mechanics
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private static final VarHandle SMATCH;
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private static final VarHandle SNEXT;
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private static final VarHandle SWAITER;
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static {
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try {
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MethodHandles.Lookup l = MethodHandles.lookup();
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SMATCH = l.findVarHandle(SNode.class, "match", SNode.class);
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SNEXT = l.findVarHandle(SNode.class, "next", SNode.class);
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SWAITER = l.findVarHandle(SNode.class, "waiter", Thread.class);
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} catch (ReflectiveOperationException e) {
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throw new ExceptionInInitializerError(e);
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}
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}
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}
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/** The head (top) of the stack */
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volatile SNode head;
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boolean casHead(SNode h, SNode nh) {
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return h == head &&
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SHEAD.compareAndSet(this, h, nh);
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}
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/**
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* Creates or resets fields of a node. Called only from transfer
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* where the node to push on stack is lazily created and
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* reused when possible to help reduce intervals between reads
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* and CASes of head and to avoid surges of garbage when CASes
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* to push nodes fail due to contention.
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*/
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static SNode snode(SNode s, Object e, SNode next, int mode) {
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if (s == null) s = new SNode(e);
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s.mode = mode;
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s.next = next;
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return s;
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}
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/**
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* Puts or takes an item.
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*/
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@SuppressWarnings("unchecked")
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E transfer(E e, boolean timed, long nanos) {
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/*
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* Basic algorithm is to loop trying one of three actions:
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*
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* 1. If apparently empty or already containing nodes of same
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* mode, try to push node on stack and wait for a match,
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* returning it, or null if cancelled.
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*
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* 2. If apparently containing node of complementary mode,
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* try to push a fulfilling node on to stack, match
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* with corresponding waiting node, pop both from
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* stack, and return matched item. The matching or
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* unlinking might not actually be necessary because of
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* other threads performing action 3:
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*
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* 3. If top of stack already holds another fulfilling node,
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* help it out by doing its match and/or pop
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* operations, and then continue. The code for helping
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* is essentially the same as for fulfilling, except
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* that it doesn't return the item.
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*/
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SNode s = null; // constructed/reused as needed
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int mode = (e == null) ? REQUEST : DATA;
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for (;;) {
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SNode h = head;
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if (h == null || h.mode == mode) { // empty or same-mode
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if (timed && nanos <= 0L) { // can't wait
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if (h != null && h.isCancelled())
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casHead(h, h.next); // pop cancelled node
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else
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return null;
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} else if (casHead(h, s = snode(s, e, h, mode))) {
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long deadline = timed ? System.nanoTime() + nanos : 0L;
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Thread w = Thread.currentThread();
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int stat = -1; // -1: may yield, +1: park, else 0
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SNode m; // await fulfill or cancel
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while ((m = s.match) == null) {
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if ((timed &&
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(nanos = deadline - System.nanoTime()) <= 0) ||
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w.isInterrupted()) {
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if (s.tryCancel()) {
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clean(s); // wait cancelled
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return null;
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}
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} else if ((m = s.match) != null) {
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break; // recheck
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} else if (stat <= 0) {
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if (stat < 0 && h == null && head == s) {
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stat = 0; // yield once if was empty
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Thread.yield();
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} else {
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stat = 1;
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s.waiter = w; // enable signal
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}
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} else if (!timed) {
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LockSupport.setCurrentBlocker(this);
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try {
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ForkJoinPool.managedBlock(s);
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} catch (InterruptedException cannotHappen) { }
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LockSupport.setCurrentBlocker(null);
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} else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
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LockSupport.parkNanos(this, nanos);
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}
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if (stat == 1)
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s.forgetWaiter();
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Object result = (mode == REQUEST) ? m.item : s.item;
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if (h != null && h.next == s)
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casHead(h, s.next); // help fulfiller
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return (E) result;
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}
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} else if (!isFulfilling(h.mode)) { // try to fulfill
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if (h.isCancelled()) // already cancelled
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casHead(h, h.next); // pop and retry
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else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
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for (;;) { // loop until matched or waiters disappear
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SNode m = s.next; // m is s's match
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if (m == null) { // all waiters are gone
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casHead(s, null); // pop fulfill node
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s = null; // use new node next time
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break; // restart main loop
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}
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SNode mn = m.next;
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if (m.tryMatch(s)) {
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casHead(s, mn); // pop both s and m
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return (E) ((mode == REQUEST) ? m.item : s.item);
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} else // lost match
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s.casNext(m, mn); // help unlink
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}
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}
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} else { // help a fulfiller
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SNode m = h.next; // m is h's match
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if (m == null) // waiter is gone
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casHead(h, null); // pop fulfilling node
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else {
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SNode mn = m.next;
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if (m.tryMatch(h)) // help match
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casHead(h, mn); // pop both h and m
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else // lost match
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h.casNext(m, mn); // help unlink
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final Object xferLifo(Object e, long ns) {
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boolean haveData = (e != null);
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Object m; // the match or e if none
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outer: for (DualNode s = null, p = head;;) {
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while (p != null) {
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boolean isData; DualNode n, u; // help collapse
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if ((isData = p.isData) != ((m = p.item) != null))
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p = (p == (u = cmpExHead(p, (n = p.next)))) ? n : u;
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else if (isData == haveData) // same mode; push below
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break;
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else if (p.cmpExItem(m, e) != m)
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p = head; // missed; restart
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else { // matched complementary node
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Thread w = p.waiter;
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cmpExHead(p, p.next);
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LockSupport.unpark(w);
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break outer;
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}
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}
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}
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}
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/**
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* Unlinks s from the stack.
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*/
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void clean(SNode s) {
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s.item = null; // forget item
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s.forgetWaiter();
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/*
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* At worst we may need to traverse entire stack to unlink
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* s. If there are multiple concurrent calls to clean, we
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* might not see s if another thread has already removed
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* it. But we can stop when we see any node known to
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* follow s. We use s.next unless it too is cancelled, in
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* which case we try the node one past. We don't check any
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* further because we don't want to doubly traverse just to
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* find sentinel.
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*/
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SNode past = s.next;
|
||||
if (past != null && past.isCancelled())
|
||||
past = past.next;
|
||||
|
||||
// Absorb cancelled nodes at head
|
||||
SNode p;
|
||||
while ((p = head) != null && p != past && p.isCancelled())
|
||||
casHead(p, p.next);
|
||||
|
||||
// Unsplice embedded nodes
|
||||
while (p != null && p != past) {
|
||||
SNode n = p.next;
|
||||
if (n != null && n.isCancelled())
|
||||
p.casNext(n, n.next);
|
||||
else
|
||||
p = n;
|
||||
}
|
||||
}
|
||||
|
||||
// VarHandle mechanics
|
||||
private static final VarHandle SHEAD;
|
||||
static {
|
||||
try {
|
||||
MethodHandles.Lookup l = MethodHandles.lookup();
|
||||
SHEAD = l.findVarHandle(TransferStack.class, "head", SNode.class);
|
||||
} catch (ReflectiveOperationException e) {
|
||||
throw new ExceptionInInitializerError(e);
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
/** Dual Queue */
|
||||
static final class TransferQueue<E> extends Transferer<E> {
|
||||
/*
|
||||
* This extends Scherer-Scott dual queue algorithm, differing,
|
||||
* among other ways, by using modes within nodes rather than
|
||||
* marked pointers. The algorithm is a little simpler than
|
||||
* that for stacks because fulfillers do not need explicit
|
||||
* nodes, and matching is done by CAS'ing QNode.item field
|
||||
* from non-null to null (for put) or vice versa (for take).
|
||||
*/
|
||||
|
||||
/** Node class for TransferQueue. */
|
||||
static final class QNode implements ForkJoinPool.ManagedBlocker {
|
||||
volatile QNode next; // next node in queue
|
||||
volatile Object item; // CAS'ed to or from null
|
||||
volatile Thread waiter; // to control park/unpark
|
||||
final boolean isData;
|
||||
|
||||
QNode(Object item, boolean isData) {
|
||||
this.item = item;
|
||||
this.isData = isData;
|
||||
}
|
||||
|
||||
boolean casNext(QNode cmp, QNode val) {
|
||||
return next == cmp &&
|
||||
QNEXT.compareAndSet(this, cmp, val);
|
||||
}
|
||||
|
||||
boolean casItem(Object cmp, Object val) {
|
||||
return item == cmp &&
|
||||
QITEM.compareAndSet(this, cmp, val);
|
||||
}
|
||||
|
||||
/**
|
||||
* Tries to cancel by CAS'ing ref to this as item.
|
||||
*/
|
||||
boolean tryCancel(Object cmp) {
|
||||
return QITEM.compareAndSet(this, cmp, this);
|
||||
}
|
||||
|
||||
boolean isCancelled() {
|
||||
return item == this;
|
||||
}
|
||||
|
||||
/**
|
||||
* Returns true if this node is known to be off the queue
|
||||
* because its next pointer has been forgotten due to
|
||||
* an advanceHead operation.
|
||||
*/
|
||||
boolean isOffList() {
|
||||
return next == this;
|
||||
}
|
||||
|
||||
void forgetWaiter() {
|
||||
QWAITER.setOpaque(this, null);
|
||||
}
|
||||
|
||||
boolean isFulfilled() {
|
||||
Object x;
|
||||
return isData == ((x = item) == null) || x == this;
|
||||
}
|
||||
|
||||
public final boolean isReleasable() {
|
||||
Object x;
|
||||
return isData == ((x = item) == null) || x == this ||
|
||||
Thread.currentThread().isInterrupted();
|
||||
}
|
||||
|
||||
public final boolean block() {
|
||||
while (!isReleasable()) LockSupport.park();
|
||||
return true;
|
||||
}
|
||||
|
||||
// VarHandle mechanics
|
||||
private static final VarHandle QITEM;
|
||||
private static final VarHandle QNEXT;
|
||||
private static final VarHandle QWAITER;
|
||||
static {
|
||||
try {
|
||||
MethodHandles.Lookup l = MethodHandles.lookup();
|
||||
QITEM = l.findVarHandle(QNode.class, "item", Object.class);
|
||||
QNEXT = l.findVarHandle(QNode.class, "next", QNode.class);
|
||||
QWAITER = l.findVarHandle(QNode.class, "waiter", Thread.class);
|
||||
} catch (ReflectiveOperationException e) {
|
||||
throw new ExceptionInInitializerError(e);
|
||||
if (ns == 0L) { // no match, no wait
|
||||
m = e;
|
||||
break;
|
||||
}
|
||||
if (s == null) // try to push node and wait
|
||||
s = new DualNode(e, haveData);
|
||||
s.next = p;
|
||||
if (p == (p = cmpExHead(p, s))) {
|
||||
if ((m = s.await(e, ns, this, // spin if (nearly) empty
|
||||
p == null || p.waiter == null)) == e)
|
||||
unspliceLifo(s); // cancelled
|
||||
break;
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
/** Head of queue */
|
||||
transient volatile QNode head;
|
||||
/** Tail of queue */
|
||||
transient volatile QNode tail;
|
||||
/**
|
||||
* Reference to a cancelled node that might not yet have been
|
||||
* unlinked from queue because it was the last inserted node
|
||||
* when it was cancelled.
|
||||
*/
|
||||
transient volatile QNode cleanMe;
|
||||
|
||||
TransferQueue() {
|
||||
QNode h = new QNode(null, false); // initialize to dummy node.
|
||||
head = h;
|
||||
tail = h;
|
||||
return m;
|
||||
}
|
||||
|
||||
/**
|
||||
* Tries to cas nh as new head; if successful, unlink
|
||||
* old head's next node to avoid garbage retention.
|
||||
* Unlinks node s. Same idea as Fifo version.
|
||||
*/
|
||||
void advanceHead(QNode h, QNode nh) {
|
||||
if (h == head &&
|
||||
QHEAD.compareAndSet(this, h, nh))
|
||||
h.next = h; // forget old next
|
||||
}
|
||||
|
||||
/**
|
||||
* Tries to cas nt as new tail.
|
||||
*/
|
||||
void advanceTail(QNode t, QNode nt) {
|
||||
if (tail == t)
|
||||
QTAIL.compareAndSet(this, t, nt);
|
||||
}
|
||||
|
||||
/**
|
||||
* Tries to CAS cleanMe slot.
|
||||
*/
|
||||
boolean casCleanMe(QNode cmp, QNode val) {
|
||||
return cleanMe == cmp &&
|
||||
QCLEANME.compareAndSet(this, cmp, val);
|
||||
}
|
||||
|
||||
/**
|
||||
* Puts or takes an item.
|
||||
*/
|
||||
@SuppressWarnings("unchecked")
|
||||
E transfer(E e, boolean timed, long nanos) {
|
||||
/* Basic algorithm is to loop trying to take either of
|
||||
* two actions:
|
||||
*
|
||||
* 1. If queue apparently empty or holding same-mode nodes,
|
||||
* try to add node to queue of waiters, wait to be
|
||||
* fulfilled (or cancelled) and return matching item.
|
||||
*
|
||||
* 2. If queue apparently contains waiting items, and this
|
||||
* call is of complementary mode, try to fulfill by CAS'ing
|
||||
* item field of waiting node and dequeuing it, and then
|
||||
* returning matching item.
|
||||
*
|
||||
* In each case, along the way, check for and try to help
|
||||
* advance head and tail on behalf of other stalled/slow
|
||||
* threads.
|
||||
*
|
||||
* The loop starts off with a null check guarding against
|
||||
* seeing uninitialized head or tail values. This never
|
||||
* happens in current SynchronousQueue, but could if
|
||||
* callers held non-volatile/final ref to the
|
||||
* transferer. The check is here anyway because it places
|
||||
* null checks at top of loop, which is usually faster
|
||||
* than having them implicitly interspersed.
|
||||
*/
|
||||
|
||||
QNode s = null; // constructed/reused as needed
|
||||
boolean isData = (e != null);
|
||||
for (;;) {
|
||||
QNode t = tail, h = head, m, tn; // m is node to fulfill
|
||||
if (t == null || h == null)
|
||||
; // inconsistent
|
||||
else if (h == t || t.isData == isData) { // empty or same-mode
|
||||
if (t != tail) // inconsistent
|
||||
;
|
||||
else if ((tn = t.next) != null) // lagging tail
|
||||
advanceTail(t, tn);
|
||||
else if (timed && nanos <= 0L) // can't wait
|
||||
return null;
|
||||
else if (t.casNext(null, (s != null) ? s :
|
||||
(s = new QNode(e, isData)))) {
|
||||
advanceTail(t, s);
|
||||
long deadline = timed ? System.nanoTime() + nanos : 0L;
|
||||
Thread w = Thread.currentThread();
|
||||
int stat = -1; // same idea as TransferStack
|
||||
Object item;
|
||||
while ((item = s.item) == e) {
|
||||
if ((timed &&
|
||||
(nanos = deadline - System.nanoTime()) <= 0) ||
|
||||
w.isInterrupted()) {
|
||||
if (s.tryCancel(e)) {
|
||||
clean(t, s);
|
||||
return null;
|
||||
}
|
||||
} else if ((item = s.item) != e) {
|
||||
break; // recheck
|
||||
} else if (stat <= 0) {
|
||||
if (t.next == s) {
|
||||
if (stat < 0 && t.isFulfilled()) {
|
||||
stat = 0; // yield once if first
|
||||
Thread.yield();
|
||||
}
|
||||
else {
|
||||
stat = 1;
|
||||
s.waiter = w;
|
||||
}
|
||||
}
|
||||
} else if (!timed) {
|
||||
LockSupport.setCurrentBlocker(this);
|
||||
try {
|
||||
ForkJoinPool.managedBlock(s);
|
||||
} catch (InterruptedException cannotHappen) { }
|
||||
LockSupport.setCurrentBlocker(null);
|
||||
}
|
||||
else if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
|
||||
LockSupport.parkNanos(this, nanos);
|
||||
}
|
||||
if (stat == 1)
|
||||
s.forgetWaiter();
|
||||
if (!s.isOffList()) { // not already unlinked
|
||||
advanceHead(t, s); // unlink if head
|
||||
if (item != null) // and forget fields
|
||||
s.item = s;
|
||||
}
|
||||
return (item != null) ? (E)item : e;
|
||||
}
|
||||
|
||||
} else if ((m = h.next) != null && t == tail && h == head) {
|
||||
Thread waiter;
|
||||
Object x = m.item;
|
||||
boolean fulfilled = ((isData == (x == null)) &&
|
||||
x != m && m.casItem(x, e));
|
||||
advanceHead(h, m); // (help) dequeue
|
||||
if (fulfilled) {
|
||||
if ((waiter = m.waiter) != null)
|
||||
LockSupport.unpark(waiter);
|
||||
return (x != null) ? (E)x : e;
|
||||
}
|
||||
}
|
||||
private void unspliceLifo(DualNode s) {
|
||||
boolean seen = false; // try removing by collapsing head
|
||||
DualNode p = head;
|
||||
for (DualNode f, u; p != null && p.matched();) {
|
||||
if (p == s)
|
||||
seen = true;
|
||||
p = (p == (u = cmpExHead(p, (f = p.next)))) ? f : u;
|
||||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* Gets rid of cancelled node s with original predecessor pred.
|
||||
*/
|
||||
void clean(QNode pred, QNode s) {
|
||||
s.forgetWaiter();
|
||||
/*
|
||||
* At any given time, exactly one node on list cannot be
|
||||
* deleted -- the last inserted node. To accommodate this,
|
||||
* if we cannot delete s, we save its predecessor as
|
||||
* "cleanMe", deleting the previously saved version
|
||||
* first. At least one of node s or the node previously
|
||||
* saved can always be deleted, so this always terminates.
|
||||
*/
|
||||
while (pred.next == s) { // Return early if already unlinked
|
||||
QNode h = head;
|
||||
QNode hn = h.next; // Absorb cancelled first node as head
|
||||
if (hn != null && hn.isCancelled()) {
|
||||
advanceHead(h, hn);
|
||||
continue;
|
||||
if (p != null && !seen && sweepNow()) { // occasionally sweep
|
||||
for (DualNode f, n, u; p != null && (f = p.next) != null; ) {
|
||||
p = (!f.matched() ? f :
|
||||
f == (u = p.cmpExNext(f, n = f.next)) ? n : u);
|
||||
}
|
||||
QNode t = tail; // Ensure consistent read for tail
|
||||
if (t == h)
|
||||
return;
|
||||
QNode tn = t.next;
|
||||
if (t != tail)
|
||||
continue;
|
||||
if (tn != null) {
|
||||
advanceTail(t, tn);
|
||||
continue;
|
||||
}
|
||||
if (s != t) { // If not tail, try to unsplice
|
||||
QNode sn = s.next;
|
||||
if (sn == s || pred.casNext(s, sn))
|
||||
return;
|
||||
}
|
||||
QNode dp = cleanMe;
|
||||
if (dp != null) { // Try unlinking previous cancelled node
|
||||
QNode d = dp.next;
|
||||
QNode dn;
|
||||
if (d == null || // d is gone or
|
||||
d == dp || // d is off list or
|
||||
!d.isCancelled() || // d not cancelled or
|
||||
(d != t && // d not tail and
|
||||
(dn = d.next) != null && // has successor
|
||||
dn != d && // that is on list
|
||||
dp.casNext(d, dn))) // d unspliced
|
||||
casCleanMe(dp, null);
|
||||
if (dp == pred)
|
||||
return; // s is already saved node
|
||||
} else if (casCleanMe(null, pred))
|
||||
return; // Postpone cleaning s
|
||||
}
|
||||
}
|
||||
|
||||
// VarHandle mechanics
|
||||
private static final VarHandle QHEAD;
|
||||
private static final VarHandle QTAIL;
|
||||
private static final VarHandle QCLEANME;
|
||||
static {
|
||||
try {
|
||||
MethodHandles.Lookup l = MethodHandles.lookup();
|
||||
QHEAD = l.findVarHandle(TransferQueue.class, "head",
|
||||
QNode.class);
|
||||
QTAIL = l.findVarHandle(TransferQueue.class, "tail",
|
||||
QNode.class);
|
||||
QCLEANME = l.findVarHandle(TransferQueue.class, "cleanMe",
|
||||
QNode.class);
|
||||
} catch (ReflectiveOperationException e) {
|
||||
throw new ExceptionInInitializerError(e);
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* The transferer. Set only in constructor, but cannot be declared
|
||||
* as final without further complicating serialization. Since
|
||||
* this is accessed only at most once per public method, there
|
||||
* isn't a noticeable performance penalty for using volatile
|
||||
* instead of final here.
|
||||
* The transferer. (See below about serialization.)
|
||||
*/
|
||||
private transient volatile Transferer<E> transferer;
|
||||
private final transient Transferer<E> transferer;
|
||||
|
||||
private final transient boolean fair;
|
||||
|
||||
/** Invokes fair or lifo transfer */
|
||||
private Object xfer(Object e, long nanos) {
|
||||
Transferer<E> x = transferer;
|
||||
return (fair) ? x.xfer(e, nanos) : x.xferLifo(e, nanos);
|
||||
}
|
||||
|
||||
/**
|
||||
* Creates a {@code SynchronousQueue} with nonfair access policy.
|
||||
@ -824,7 +247,8 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
|
||||
* access; otherwise the order is unspecified.
|
||||
*/
|
||||
public SynchronousQueue(boolean fair) {
|
||||
transferer = fair ? new TransferQueue<E>() : new TransferStack<E>();
|
||||
this.fair = fair;
|
||||
transferer = new Transferer<E>();
|
||||
}
|
||||
|
||||
/**
|
||||
@ -835,11 +259,13 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
|
||||
* @throws NullPointerException {@inheritDoc}
|
||||
*/
|
||||
public void put(E e) throws InterruptedException {
|
||||
if (e == null) throw new NullPointerException();
|
||||
if (transferer.transfer(e, false, 0) == null) {
|
||||
Thread.interrupted();
|
||||
throw new InterruptedException();
|
||||
Objects.requireNonNull(e);
|
||||
if (!Thread.interrupted()) {
|
||||
if (xfer(e, Long.MAX_VALUE) == null)
|
||||
return;
|
||||
Thread.interrupted(); // failure possible only due to interrupt
|
||||
}
|
||||
throw new InterruptedException();
|
||||
}
|
||||
|
||||
/**
|
||||
@ -853,8 +279,9 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
|
||||
*/
|
||||
public boolean offer(E e, long timeout, TimeUnit unit)
|
||||
throws InterruptedException {
|
||||
if (e == null) throw new NullPointerException();
|
||||
if (transferer.transfer(e, true, unit.toNanos(timeout)) != null)
|
||||
Objects.requireNonNull(e);
|
||||
long nanos = Math.max(unit.toNanos(timeout), 0L);
|
||||
if (xfer(e, nanos) == null)
|
||||
return true;
|
||||
if (!Thread.interrupted())
|
||||
return false;
|
||||
@ -871,8 +298,8 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
|
||||
* @throws NullPointerException if the specified element is null
|
||||
*/
|
||||
public boolean offer(E e) {
|
||||
if (e == null) throw new NullPointerException();
|
||||
return transferer.transfer(e, true, 0) != null;
|
||||
Objects.requireNonNull(e);
|
||||
return xfer(e, 0L) == null;
|
||||
}
|
||||
|
||||
/**
|
||||
@ -882,11 +309,14 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
|
||||
* @return the head of this queue
|
||||
* @throws InterruptedException {@inheritDoc}
|
||||
*/
|
||||
@SuppressWarnings("unchecked")
|
||||
public E take() throws InterruptedException {
|
||||
E e = transferer.transfer(null, false, 0);
|
||||
if (e != null)
|
||||
return e;
|
||||
Thread.interrupted();
|
||||
Object e;
|
||||
if (!Thread.interrupted()) {
|
||||
if ((e = xfer(null, Long.MAX_VALUE)) != null)
|
||||
return (E) e;
|
||||
Thread.interrupted();
|
||||
}
|
||||
throw new InterruptedException();
|
||||
}
|
||||
|
||||
@ -899,10 +329,12 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
|
||||
* specified waiting time elapses before an element is present
|
||||
* @throws InterruptedException {@inheritDoc}
|
||||
*/
|
||||
@SuppressWarnings("unchecked")
|
||||
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
|
||||
E e = transferer.transfer(null, true, unit.toNanos(timeout));
|
||||
if (e != null || !Thread.interrupted())
|
||||
return e;
|
||||
Object e;
|
||||
long nanos = Math.max(unit.toNanos(timeout), 0L);
|
||||
if ((e = xfer(null, nanos)) != null || !Thread.interrupted())
|
||||
return (E) e;
|
||||
throw new InterruptedException();
|
||||
}
|
||||
|
||||
@ -913,8 +345,9 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
|
||||
* @return the head of this queue, or {@code null} if no
|
||||
* element is available
|
||||
*/
|
||||
@SuppressWarnings("unchecked")
|
||||
public E poll() {
|
||||
return transferer.transfer(null, true, 0);
|
||||
return (E) xfer(null, 0L);
|
||||
}
|
||||
|
||||
/**
|
||||
@ -1104,11 +537,13 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
|
||||
}
|
||||
|
||||
/*
|
||||
* To cope with serialization strategy in the 1.5 version of
|
||||
* SynchronousQueue, we declare some unused classes and fields
|
||||
* that exist solely to enable serializability across versions.
|
||||
* These fields are never used, so are initialized only if this
|
||||
* object is ever serialized or deserialized.
|
||||
* To cope with serialization across multiple implementation
|
||||
* overhauls, we declare some unused classes and fields that exist
|
||||
* solely to enable serializability across versions. These fields
|
||||
* are never used, so are initialized only if this object is ever
|
||||
* serialized. We use readResolve to replace a deserialized queue
|
||||
* with a fresh one. Note that no queue elements are serialized,
|
||||
* since any existing ones are only transient.
|
||||
*/
|
||||
|
||||
@SuppressWarnings("serial")
|
||||
@ -1130,7 +565,6 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
|
||||
*/
|
||||
private void writeObject(java.io.ObjectOutputStream s)
|
||||
throws java.io.IOException {
|
||||
boolean fair = transferer instanceof TransferQueue;
|
||||
if (fair) {
|
||||
qlock = new ReentrantLock(true);
|
||||
waitingProducers = new FifoWaitQueue();
|
||||
@ -1145,24 +579,10 @@ public class SynchronousQueue<E> extends AbstractQueue<E>
|
||||
}
|
||||
|
||||
/**
|
||||
* Reconstitutes this queue from a stream (that is, deserializes it).
|
||||
* @param s the stream
|
||||
* @throws ClassNotFoundException if the class of a serialized object
|
||||
* could not be found
|
||||
* @throws java.io.IOException if an I/O error occurs
|
||||
* Replaces a deserialized SynchronousQueue with a fresh one with
|
||||
* the associated fairness
|
||||
*/
|
||||
private void readObject(java.io.ObjectInputStream s)
|
||||
throws java.io.IOException, ClassNotFoundException {
|
||||
s.defaultReadObject();
|
||||
if (waitingProducers instanceof FifoWaitQueue)
|
||||
transferer = new TransferQueue<E>();
|
||||
else
|
||||
transferer = new TransferStack<E>();
|
||||
}
|
||||
|
||||
static {
|
||||
// Reduce the risk of rare disastrous classloading in first call to
|
||||
// LockSupport.park: https://bugs.openjdk.org/browse/JDK-8074773
|
||||
Class<?> ensureLoaded = LockSupport.class;
|
||||
private Object readResolve() {
|
||||
return new SynchronousQueue<E>(waitingProducers instanceof FifoWaitQueue);
|
||||
}
|
||||
}
|
||||
|
||||
@ -59,18 +59,24 @@ import java.util.function.Consumer;
|
||||
|
||||
@Test
|
||||
public class WhiteBox {
|
||||
|
||||
final ThreadLocalRandom rnd = ThreadLocalRandom.current();
|
||||
final VarHandle HEAD, TAIL, ITEM, NEXT;
|
||||
|
||||
public WhiteBox() throws ReflectiveOperationException {
|
||||
Class<?> qClass = LinkedTransferQueue.class;
|
||||
Class<?> nodeClass = Class.forName(qClass.getName() + "$Node");
|
||||
MethodHandles.Lookup lookup
|
||||
= MethodHandles.privateLookupIn(qClass, MethodHandles.lookup());
|
||||
HEAD = lookup.findVarHandle(qClass, "head", nodeClass);
|
||||
TAIL = lookup.findVarHandle(qClass, "tail", nodeClass);
|
||||
NEXT = lookup.findVarHandle(nodeClass, "next", nodeClass);
|
||||
ITEM = lookup.findVarHandle(nodeClass, "item", Object.class);
|
||||
public WhiteBox() throws Throwable { // throws ReflectiveOperationException {
|
||||
try {
|
||||
Class<?> qClass = LinkedTransferQueue.class;
|
||||
Class<?> nodeClass = Class.forName(qClass.getName() + "$DualNode");
|
||||
MethodHandles.Lookup lookup
|
||||
= MethodHandles.privateLookupIn(qClass, MethodHandles.lookup());
|
||||
HEAD = lookup.findVarHandle(qClass, "head", nodeClass);
|
||||
TAIL = lookup.findVarHandle(qClass, "tail", nodeClass);
|
||||
NEXT = lookup.findVarHandle(nodeClass, "next", nodeClass);
|
||||
ITEM = lookup.findVarHandle(nodeClass, "item", Object.class);
|
||||
} catch (Throwable ex) {
|
||||
ex.printStackTrace();
|
||||
throw ex;
|
||||
}
|
||||
}
|
||||
|
||||
Object head(LinkedTransferQueue q) { return HEAD.getVolatile(q); }
|
||||
@ -78,6 +84,16 @@ public class WhiteBox {
|
||||
Object item(Object node) { return ITEM.getVolatile(node); }
|
||||
Object next(Object node) { return NEXT.getVolatile(node); }
|
||||
|
||||
/*
|
||||
* Modified for jdk22: Accommodate lazy initialization, so counts
|
||||
* may vary by 1, and some nodes become headers vs unlinked,
|
||||
* compared to previous versions.
|
||||
*/
|
||||
|
||||
static void checkCount(int val, int expect) {
|
||||
assertTrue(val == expect || val == expect - 1);
|
||||
}
|
||||
|
||||
int nodeCount(LinkedTransferQueue q) {
|
||||
int i = 0;
|
||||
for (Object p = head(q); p != null; ) {
|
||||
@ -124,13 +140,15 @@ public class WhiteBox {
|
||||
public void addRemove() {
|
||||
LinkedTransferQueue q = new LinkedTransferQueue();
|
||||
assertInvariants(q);
|
||||
assertNull(next(head(q)));
|
||||
assertNull(item(head(q)));
|
||||
if (head(q) != null) {
|
||||
assertNull(next(head(q)));
|
||||
assertNull(item(head(q)));
|
||||
}
|
||||
q.add(1);
|
||||
assertEquals(nodeCount(q), 2);
|
||||
checkCount(nodeCount(q), 2);
|
||||
assertInvariants(q);
|
||||
q.remove(1);
|
||||
assertEquals(nodeCount(q), 1);
|
||||
checkCount(nodeCount(q), 1);
|
||||
assertInvariants(q);
|
||||
}
|
||||
|
||||
@ -158,12 +176,12 @@ public class WhiteBox {
|
||||
Object oldHead;
|
||||
int n = 1 + rnd.nextInt(5);
|
||||
for (int i = 0; i < n; i++) q.add(i);
|
||||
assertEquals(nodeCount(q), n + 1);
|
||||
checkCount(nodeCount(q), n + 1);
|
||||
oldHead = head(q);
|
||||
traversalAction.accept(q);
|
||||
assertInvariants(q);
|
||||
assertEquals(nodeCount(q), n);
|
||||
assertIsSelfLinked(oldHead);
|
||||
checkCount(nodeCount(q), n);
|
||||
// assertIsSelfLinked(oldHead);
|
||||
}
|
||||
|
||||
@Test(dataProvider = "traversalActions")
|
||||
@ -204,7 +222,7 @@ public class WhiteBox {
|
||||
|
||||
int c = nodeCount(q);
|
||||
traversalAction.accept(q);
|
||||
assertEquals(nodeCount(q), c - 1);
|
||||
checkCount(nodeCount(q), c - 1);
|
||||
|
||||
assertSame(next(p0), p4);
|
||||
assertSame(next(p1), p4);
|
||||
@ -217,7 +235,7 @@ public class WhiteBox {
|
||||
traversalAction.accept(q);
|
||||
assertSame(next(p4), p5);
|
||||
assertNull(next(p5));
|
||||
assertEquals(nodeCount(q), c - 1);
|
||||
checkCount(nodeCount(q), c - 1);
|
||||
}
|
||||
|
||||
/**
|
||||
@ -228,7 +246,7 @@ public class WhiteBox {
|
||||
public void traversalOperationsCollapseRandomNodes(
|
||||
Consumer<LinkedTransferQueue> traversalAction) {
|
||||
LinkedTransferQueue q = new LinkedTransferQueue();
|
||||
int n = rnd.nextInt(6);
|
||||
int n = 1 + rnd.nextInt(6);
|
||||
for (int i = 0; i < n; i++) q.add(i);
|
||||
ArrayList nulledOut = new ArrayList();
|
||||
for (Object p = head(q); p != null; p = next(p))
|
||||
@ -238,7 +256,7 @@ public class WhiteBox {
|
||||
}
|
||||
traversalAction.accept(q);
|
||||
int c = nodeCount(q);
|
||||
assertEquals(q.size(), c - (q.contains(n - 1) ? 0 : 1));
|
||||
checkCount(c - (q.contains(n - 1) ? 0 : 1), q.size() + 1);
|
||||
for (int i = 0; i < n; i++)
|
||||
assertTrue(nulledOut.contains(i) ^ q.contains(i));
|
||||
}
|
||||
@ -263,7 +281,7 @@ public class WhiteBox {
|
||||
int n = 1 + rnd.nextInt(5);
|
||||
for (int i = 0; i < n; i++) q.add(i);
|
||||
bulkRemovalAction.accept(q);
|
||||
assertEquals(nodeCount(q), 1);
|
||||
checkCount(nodeCount(q), 1);
|
||||
assertInvariants(q);
|
||||
}
|
||||
|
||||
@ -289,13 +307,13 @@ public class WhiteBox {
|
||||
LinkedTransferQueue q = new LinkedTransferQueue();
|
||||
int n = 1 + rnd.nextInt(5);
|
||||
for (int i = 0; i < n; i++) q.add(i);
|
||||
assertEquals(nodeCount(q), n + 1);
|
||||
checkCount(nodeCount(q), n + 1);
|
||||
for (int i = 0; i < n; i++) {
|
||||
int c = nodeCount(q);
|
||||
boolean slack = item(head(q)) == null;
|
||||
if (slack) assertNotNull(item(next(head(q))));
|
||||
pollAction.accept(q);
|
||||
assertEquals(nodeCount(q), q.isEmpty() ? 1 : c - (slack ? 2 : 0));
|
||||
checkCount(nodeCount(q), q.isEmpty() ? 1 : c - (slack ? 2 : 0));
|
||||
}
|
||||
assertInvariants(q);
|
||||
}
|
||||
@ -318,11 +336,12 @@ public class WhiteBox {
|
||||
LinkedTransferQueue q = new LinkedTransferQueue();
|
||||
int n = 1 + rnd.nextInt(9);
|
||||
for (int i = 0; i < n; i++) {
|
||||
boolean slack = next(tail(q)) != null;
|
||||
boolean empty = (tail(q) == null);
|
||||
boolean slack = !empty && (next(tail(q)) != null);
|
||||
addAction.accept(q);
|
||||
if (slack)
|
||||
assertNull(next(tail(q)));
|
||||
else {
|
||||
else if (!empty) {
|
||||
assertNotNull(next(tail(q)));
|
||||
assertNull(next(next(tail(q))));
|
||||
}
|
||||
@ -365,10 +384,9 @@ public class WhiteBox {
|
||||
|
||||
/** Checks conditions which should always be true. */
|
||||
void assertInvariants(LinkedTransferQueue q) {
|
||||
assertNotNull(head(q));
|
||||
assertNotNull(tail(q));
|
||||
// head is never self-linked (but tail may!)
|
||||
for (Object h; next(h = head(q)) == h; )
|
||||
assertNotSame(h, head(q)); // must be update race
|
||||
Object h;
|
||||
if ((h = head(q)) != null)
|
||||
assertNotSame(h, next(h));
|
||||
}
|
||||
}
|
||||
|
||||
Loading…
Reference in New Issue
Block a user