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	/*
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	/*
	* This file is available under and governed by the GNU General Public
	* License version 2 only, as published by the Free Software Foundation.
	* However, the following notice accompanied the original version of this
	* file:
	*
	* Written by Doug Lea with assistance from members of JCP JSR-166
	* Expert Group and released to the public domain, as explained at
	* http://creativecommons.org/publicdomain/zero/1.0/
	*/

	package java.util.concurrent.locks;

	import jdk.internal.misc.Unsafe;

	/**
	* Basic thread blocking primitives for creating locks and other
	* synchronization classes.
	*
	* <p>This class associates, with each thread that uses it, a permit
	* (in the sense of the {@link java.util.concurrent.Semaphore
	* Semaphore} class). A call to {@code park} will return immediately
	* if the permit is available, consuming it in the process; otherwise
	* it <em>may</em> block. A call to {@code unpark} makes the permit
	* available, if it was not already available. (Unlike with Semaphores
	* though, permits do not accumulate. There is at most one.)
	* Reliable usage requires the use of volatile (or atomic) variables
	* to control when to park or unpark. Orderings of calls to these
	* methods are maintained with respect to volatile variable accesses,
	* but not necessarily non-volatile variable accesses.
	*
	* <p>Methods {@code park} and {@code unpark} provide efficient
	* means of blocking and unblocking threads that do not encounter the
	* problems that cause the deprecated methods {@code Thread.suspend}
	* and {@code Thread.resume} to be unusable for such purposes: Races
	* between one thread invoking {@code park} and another thread trying
	* to {@code unpark} it will preserve liveness, due to the
	* permit. Additionally, {@code park} will return if the caller's
	* thread was interrupted, and timeout versions are supported. The
	* {@code park} method may also return at any other time, for "no
	* reason", so in general must be invoked within a loop that rechecks
	* conditions upon return. In this sense {@code park} serves as an
	* optimization of a "busy wait" that does not waste as much time
	* spinning, but must be paired with an {@code unpark} to be
	* effective.
	*
	* <p>The three forms of {@code park} each also support a
	* {@code blocker} object parameter. This object is recorded while
	* the thread is blocked to permit monitoring and diagnostic tools to
	* identify the reasons that threads are blocked. (Such tools may
	* access blockers using method {@link #getBlocker(Thread)}.)
	* The use of these forms rather than the original forms without this
	* parameter is strongly encouraged. The normal argument to supply as
	* a {@code blocker} within a lock implementation is {@code this}.
	*
	* <p>These methods are designed to be used as tools for creating
	* higher-level synchronization utilities, and are not in themselves
	* useful for most concurrency control applications. The {@code park}
	* method is designed for use only in constructions of the form:
	*
	* <pre> {@code
	* while (!canProceed()) {
	* // ensure request to unpark is visible to other threads
	* ...
	* LockSupport.park(this);
	* }}</pre>
	*
	* where no actions by the thread publishing a request to unpark,
	* prior to the call to {@code park}, entail locking or blocking.
	* Because only one permit is associated with each thread, any
	* intermediary uses of {@code park}, including implicitly via class
	* loading, could lead to an unresponsive thread (a "lost unpark").
	*
	* <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out
	* non-reentrant lock class:
	* <pre> {@code
	* class FIFOMutex {
	* private final AtomicBoolean locked = new AtomicBoolean(false);
	* private final Queue<Thread> waiters
	* = new ConcurrentLinkedQueue<>();
	*
	* public void lock() {
	* boolean wasInterrupted = false;
	* // publish current thread for unparkers
	* waiters.add(Thread.currentThread());
	*
	* // Block while not first in queue or cannot acquire lock
	* while (waiters.peek() != Thread.currentThread() \|\|
	* !locked.compareAndSet(false, true)) {
	* LockSupport.park(this);
	* // ignore interrupts while waiting
	* if (Thread.interrupted())
	* wasInterrupted = true;
	* }
	*
	* waiters.remove();
	* // ensure correct interrupt status on return
	* if (wasInterrupted)
	* Thread.currentThread().interrupt();
	* }
	*
	* public void unlock() {
	* locked.set(false);
	* LockSupport.unpark(waiters.peek());
	* }
	*
	* static {
	* // Reduce the risk of "lost unpark" due to classloading
	* Class<?> ensureLoaded = LockSupport.class;
	* }
	* }}</pre>
	*
	* @since 1.5
	*/
	public class LockSupport {
	private LockSupport() {} // Cannot be instantiated.

	private static void setBlocker(Thread t, Object arg) {
	// Even though volatile, hotspot doesn't need a write barrier here.
	U.putObject(t, PARKBLOCKER, arg);
	}

	/**
	* Makes available the permit for the given thread, if it
	* was not already available. If the thread was blocked on
	* {@code park} then it will unblock. Otherwise, its next call
	* to {@code park} is guaranteed not to block. This operation
	* is not guaranteed to have any effect at all if the given
	* thread has not been started.
	*
	* @param thread the thread to unpark, or {@code null}, in which case
	* this operation has no effect
	*/
	public static void unpark(Thread thread) {
	if (thread != null)
	U.unpark(thread);
	}

	/**
	* Disables the current thread for thread scheduling purposes unless the
	* permit is available.
	*
	* <p>If the permit is available then it is consumed and the call returns
	* immediately; otherwise
	* the current thread becomes disabled for thread scheduling
	* purposes and lies dormant until one of three things happens:
	*
	* <ul>
	* <li>Some other thread invokes {@link #unpark unpark} with the
	* current thread as the target; or
	*
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* the current thread; or
	*
	* <li>The call spuriously (that is, for no reason) returns.
	* </ul>
	*
	* <p>This method does <em>not</em> report which of these caused the
	* method to return. Callers should re-check the conditions which caused
	* the thread to park in the first place. Callers may also determine,
	* for example, the interrupt status of the thread upon return.
	*
	* @param blocker the synchronization object responsible for this
	* thread parking
	* @since 1.6
	*/
	public static void park(Object blocker) {
	Thread t = Thread.currentThread();
	setBlocker(t, blocker);
	U.park(false, 0L);
	setBlocker(t, null);
	}

	/**
	* Disables the current thread for thread scheduling purposes, for up to
	* the specified waiting time, unless the permit is available.
	*
	* <p>If the permit is available then it is consumed and the call
	* returns immediately; otherwise the current thread becomes disabled
	* for thread scheduling purposes and lies dormant until one of four
	* things happens:
	*
	* <ul>
	* <li>Some other thread invokes {@link #unpark unpark} with the
	* current thread as the target; or
	*
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* the current thread; or
	*
	* <li>The specified waiting time elapses; or
	*
	* <li>The call spuriously (that is, for no reason) returns.
	* </ul>
	*
	* <p>This method does <em>not</em> report which of these caused the
	* method to return. Callers should re-check the conditions which caused
	* the thread to park in the first place. Callers may also determine,
	* for example, the interrupt status of the thread, or the elapsed time
	* upon return.
	*
	* @param blocker the synchronization object responsible for this
	* thread parking
	* @param nanos the maximum number of nanoseconds to wait
	* @since 1.6
	*/
	public static void parkNanos(Object blocker, long nanos) {
	if (nanos > 0) {
	Thread t = Thread.currentThread();
	setBlocker(t, blocker);
	U.park(false, nanos);
	setBlocker(t, null);
	}
	}

	/**
	* Disables the current thread for thread scheduling purposes, until
	* the specified deadline, unless the permit is available.
	*
	* <p>If the permit is available then it is consumed and the call
	* returns immediately; otherwise the current thread becomes disabled
	* for thread scheduling purposes and lies dormant until one of four
	* things happens:
	*
	* <ul>
	* <li>Some other thread invokes {@link #unpark unpark} with the
	* current thread as the target; or
	*
	* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
	* current thread; or
	*
	* <li>The specified deadline passes; or
	*
	* <li>The call spuriously (that is, for no reason) returns.
	* </ul>
	*
	* <p>This method does <em>not</em> report which of these caused the
	* method to return. Callers should re-check the conditions which caused
	* the thread to park in the first place. Callers may also determine,
	* for example, the interrupt status of the thread, or the current time
	* upon return.
	*
	* @param blocker the synchronization object responsible for this
	* thread parking
	* @param deadline the absolute time, in milliseconds from the Epoch,
	* to wait until
	* @since 1.6
	*/
	public static void parkUntil(Object blocker, long deadline) {
	Thread t = Thread.currentThread();
	setBlocker(t, blocker);
	U.park(true, deadline);
	setBlocker(t, null);
	}

	/**
	* Returns the blocker object supplied to the most recent
	* invocation of a park method that has not yet unblocked, or null
	* if not blocked. The value returned is just a momentary
	* snapshot -- the thread may have since unblocked or blocked on a
	* different blocker object.
	*
	* @param t the thread
	* @return the blocker
	* @throws NullPointerException if argument is null
	* @since 1.6
	*/
	public static Object getBlocker(Thread t) {
	if (t == null)
	throw new NullPointerException();
	return U.getObjectVolatile(t, PARKBLOCKER);
	}

	/**
	* Disables the current thread for thread scheduling purposes unless the
	* permit is available.
	*
	* <p>If the permit is available then it is consumed and the call
	* returns immediately; otherwise the current thread becomes disabled
	* for thread scheduling purposes and lies dormant until one of three
	* things happens:
	*
	* <ul>
	*
	* <li>Some other thread invokes {@link #unpark unpark} with the
	* current thread as the target; or
	*
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* the current thread; or
	*
	* <li>The call spuriously (that is, for no reason) returns.
	* </ul>
	*
	* <p>This method does <em>not</em> report which of these caused the
	* method to return. Callers should re-check the conditions which caused
	* the thread to park in the first place. Callers may also determine,
	* for example, the interrupt status of the thread upon return.
	*/
	public static void park() {
	U.park(false, 0L);
	}

	/**
	* Disables the current thread for thread scheduling purposes, for up to
	* the specified waiting time, unless the permit is available.
	*
	* <p>If the permit is available then it is consumed and the call
	* returns immediately; otherwise the current thread becomes disabled
	* for thread scheduling purposes and lies dormant until one of four
	* things happens:
	*
	* <ul>
	* <li>Some other thread invokes {@link #unpark unpark} with the
	* current thread as the target; or
	*
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* the current thread; or
	*
	* <li>The specified waiting time elapses; or
	*
	* <li>The call spuriously (that is, for no reason) returns.
	* </ul>
	*
	* <p>This method does <em>not</em> report which of these caused the
	* method to return. Callers should re-check the conditions which caused
	* the thread to park in the first place. Callers may also determine,
	* for example, the interrupt status of the thread, or the elapsed time
	* upon return.
	*
	* @param nanos the maximum number of nanoseconds to wait
	*/
	public static void parkNanos(long nanos) {
	if (nanos > 0)
	U.park(false, nanos);
	}

	/**
	* Disables the current thread for thread scheduling purposes, until
	* the specified deadline, unless the permit is available.
	*
	* <p>If the permit is available then it is consumed and the call
	* returns immediately; otherwise the current thread becomes disabled
	* for thread scheduling purposes and lies dormant until one of four
	* things happens:
	*
	* <ul>
	* <li>Some other thread invokes {@link #unpark unpark} with the
	* current thread as the target; or
	*
	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
	* the current thread; or
	*
	* <li>The specified deadline passes; or
	*
	* <li>The call spuriously (that is, for no reason) returns.
	* </ul>
	*
	* <p>This method does <em>not</em> report which of these caused the
	* method to return. Callers should re-check the conditions which caused
	* the thread to park in the first place. Callers may also determine,
	* for example, the interrupt status of the thread, or the current time
	* upon return.
	*
	* @param deadline the absolute time, in milliseconds from the Epoch,
	* to wait until
	*/
	public static void parkUntil(long deadline) {
	U.park(true, deadline);
	}

	/**
	* Returns the pseudo-randomly initialized or updated secondary seed.
	* Copied from ThreadLocalRandom due to package access restrictions.
	*/
	static final int nextSecondarySeed() {
	int r;
	Thread t = Thread.currentThread();
	if ((r = U.getInt(t, SECONDARY)) != 0) {
	r ^= r << 13; // xorshift
	r ^= r >>> 17;
	r ^= r << 5;
	}
	else if ((r = java.util.concurrent.ThreadLocalRandom.current().nextInt()) == 0)
	r = 1; // avoid zero
	U.putInt(t, SECONDARY, r);
	return r;
	}

	/**
	* Returns the thread id for the given thread. We must access
	* this directly rather than via method Thread.getId() because
	* getId() has been known to be overridden in ways that do not
	* preserve unique mappings.
	*/
	static final long getThreadId(Thread thread) {
	return U.getLong(thread, TID);
	}

	// Hotspot implementation via intrinsics API
	private static final Unsafe U = Unsafe.getUnsafe();
	private static final long PARKBLOCKER = U.objectFieldOffset
	(Thread.class, "parkBlocker");
	private static final long SECONDARY = U.objectFieldOffset
	(Thread.class, "threadLocalRandomSecondarySeed");
	private static final long TID = U.objectFieldOffset
	(Thread.class, "tid");

	}

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