/*

 * Copyright (c) 1997, 2015, Oracle and/or its affiliates. All rights reserved.

 * 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.

 *

 * 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.

 *

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 */

#ifndef SHARE_VM_GC_SHARED_GCLOCKER_HPP

#define SHARE_VM_GC_SHARED_GCLOCKER_HPP

#include "memory/allocation.hpp"

#include "utilities/globalDefinitions.hpp"

#include "utilities/macros.hpp"

class JavaThread;

// The direct lock/unlock calls do not force a collection if an unlock

// decrements the count to zero. Avoid calling these if at all possible.

class GCLocker: public AllStatic {

 private:

  // The _jni_lock_count keeps track of the number of threads that are

  // currently in a critical region.  It's only kept up to date when

  // _needs_gc is true.  The current value is computed during

  // safepointing and decremented during the slow path of GCLocker

  // unlocking.

  static volatile jint _jni_lock_count;  // number of jni active instances.

  static volatile bool _needs_gc;        // heap is filling, we need a GC

                                         // note: bool is typedef'd as jint

  static volatile bool _doing_gc;        // unlock_critical() is doing a GC

#ifdef ASSERT

  // This lock count is updated for all operations and is used to

  // validate the jni_lock_count that is computed during safepoints.

  static volatile jint _debug_jni_lock_count;

#endif

  // At a safepoint, visit all threads and count the number of active

  // critical sections.  This is used to ensure that all active

  // critical sections are exited before a new one is started.

  static void verify_critical_count() NOT_DEBUG_RETURN;

  static void jni_lock(JavaThread* thread);

  static void jni_unlock(JavaThread* thread);

  static bool is_active_internal() {

    verify_critical_count();

    return _jni_lock_count > 0;

  }

  static void log_debug_jni(const char* msg);

  static bool is_at_safepoint();

 public:

  // Accessors

  static bool is_active() {

    assert(GCLocker::is_at_safepoint(), "only read at safepoint");

    return is_active_internal();

  }

  static bool needs_gc()       { return _needs_gc;                        }

  // Shorthand

  static bool is_active_and_needs_gc() {

    // Use is_active_internal since _needs_gc can change from true to

    // false outside of a safepoint, triggering the assert in

    // is_active.

    return needs_gc() && is_active_internal();

  }

  // In debug mode track the locking state at all times

  static void increment_debug_jni_lock_count() NOT_DEBUG_RETURN;

  static void decrement_debug_jni_lock_count() NOT_DEBUG_RETURN;

  // Set the current lock count

  static void set_jni_lock_count(int count) {

    _jni_lock_count = count;

    verify_critical_count();

  }

  // Sets _needs_gc if is_active() is true. Returns is_active().

  static bool check_active_before_gc();

  // Stalls the caller (who should not be in a jni critical section)

  // until needs_gc() clears. Note however that needs_gc() may be

  // set at a subsequent safepoint and/or cleared under the

  // JNICritical_lock, so the caller may not safely assert upon

  // return from this method that "!needs_gc()" since that is

  // not a stable predicate.

  static void stall_until_clear();

  // The following two methods are used for JNI critical regions.

  // If we find that we failed to perform a GC because the GCLocker

  // was active, arrange for one as soon as possible by allowing

  // all threads in critical regions to complete, but not allowing

  // other critical regions to be entered. The reasons for that are:

  // 1) a GC request won't be starved by overlapping JNI critical

  //    region activities, which can cause unnecessary OutOfMemory errors.

  // 2) even if allocation requests can still be satisfied before GC locker

  //    becomes inactive, for example, in tenured generation possibly with

  //    heap expansion, those allocations can trigger lots of safepointing

  //    attempts (ineffective GC attempts) and require Heap_lock which

  //    slow down allocations tremendously.

  //

  // Note that critical regions can be nested in a single thread, so

  // we must allow threads already in critical regions to continue.

  //

  // JNI critical regions are the only participants in this scheme

  // because they are, by spec, well bounded while in a critical region.

  //

  // Each of the following two method is split into a fast path and a

  // slow path. JNICritical_lock is only grabbed in the slow path.

  // _needs_gc is initially false and every java thread will go

  // through the fast path, which simply increments or decrements the

  // current thread's critical count.  When GC happens at a safepoint,

  // GCLocker::is_active() is checked. Since there is no safepoint in

  // the fast path of lock_critical() and unlock_critical(), there is

  // no race condition between the fast path and GC. After _needs_gc

  // is set at a safepoint, every thread will go through the slow path

  // after the safepoint.  Since after a safepoint, each of the

  // following two methods is either entered from the method entry and

  // falls into the slow path, or is resumed from the safepoints in

  // the method, which only exist in the slow path. So when _needs_gc

  // is set, the slow path is always taken, till _needs_gc is cleared.

  inline static void lock_critical(JavaThread* thread);

  inline static void unlock_critical(JavaThread* thread);

  static address needs_gc_address() { return (address) &_needs_gc; }

};

#endif // SHARE_VM_GC_SHARED_GCLOCKER_HPP