/*

 * 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.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

#ifndef SHARE_VM_GC_SHARED_GCLOCKER_HPP

#define SHARE_VM_GC_SHARED_GCLOCKER_HPP

#include "gc/shared/collectedHeap.hpp"

#include "gc/shared/genCollectedHeap.hpp"

#include "memory/universe.hpp"

#include "oops/oop.hpp"

// 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);

 public:

  // Accessors

  static bool is_active() {

    assert(SafepointSynchronize::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.

  static void lock_critical(JavaThread* thread);

  static void unlock_critical(JavaThread* thread);

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

};

// A NoGCVerifier object can be placed in methods where one assumes that

// no garbage collection will occur. The destructor will verify this property

// unless the constructor is called with argument false (not verifygc).

//

// The check will only be done in debug mode and if verifygc true.

class NoGCVerifier: public StackObj {

 friend class PauseNoGCVerifier;

 protected:

  bool _verifygc;

  unsigned int _old_invocations;

 public:

#ifdef ASSERT

  NoGCVerifier(bool verifygc = true);

  ~NoGCVerifier();

#else

  NoGCVerifier(bool verifygc = true) {}

  ~NoGCVerifier() {}

#endif

};

// A PauseNoGCVerifier is used to temporarily pause the behavior

// of a NoGCVerifier object. If we are not in debug mode or if the

// NoGCVerifier object has a _verifygc value of false, then there

// is nothing to do.

class PauseNoGCVerifier: public StackObj {

 private:

  NoGCVerifier * _ngcv;

 public:

#ifdef ASSERT

  PauseNoGCVerifier(NoGCVerifier * ngcv);

  ~PauseNoGCVerifier();

#else

  PauseNoGCVerifier(NoGCVerifier * ngcv) {}

  ~PauseNoGCVerifier() {}

#endif

};

// A NoSafepointVerifier object will throw an assertion failure if

// the current thread passes a possible safepoint while this object is

// instantiated. A safepoint, will either be: an oop allocation, blocking

// on a Mutex or JavaLock, or executing a VM operation.

//

// If StrictSafepointChecks is turned off, it degrades into a NoGCVerifier

//

class NoSafepointVerifier : public NoGCVerifier {

 friend class PauseNoSafepointVerifier;

 private:

  bool _activated;

  Thread *_thread;

 public:

#ifdef ASSERT

  NoSafepointVerifier(bool activated = true, bool verifygc = true ) :

    NoGCVerifier(verifygc),

    _activated(activated) {

    _thread = Thread::current();

    if (_activated) {

      _thread->_allow_allocation_count++;

      _thread->_allow_safepoint_count++;

    }

  }

  ~NoSafepointVerifier() {

    if (_activated) {

      _thread->_allow_allocation_count--;

      _thread->_allow_safepoint_count--;

    }

  }

#else

  NoSafepointVerifier(bool activated = true, bool verifygc = true) : NoGCVerifier(verifygc){}

  ~NoSafepointVerifier() {}

#endif

};

// A PauseNoSafepointVerifier is used to temporarily pause the

// behavior of a NoSafepointVerifier object. If we are not in debug

// mode then there is nothing to do. If the NoSafepointVerifier

// object has an _activated value of false, then there is nothing to

// do for safepoint and allocation checking, but there may still be

// something to do for the underlying NoGCVerifier object.

class PauseNoSafepointVerifier : public PauseNoGCVerifier {

 private:

  NoSafepointVerifier * _nsv;

 public:

#ifdef ASSERT

  PauseNoSafepointVerifier(NoSafepointVerifier * nsv)

    : PauseNoGCVerifier(nsv) {

    _nsv = nsv;

    if (_nsv->_activated) {

      _nsv->_thread->_allow_allocation_count--;

      _nsv->_thread->_allow_safepoint_count--;

    }

  }

  ~PauseNoSafepointVerifier() {

    if (_nsv->_activated) {

      _nsv->_thread->_allow_allocation_count++;

      _nsv->_thread->_allow_safepoint_count++;

    }

  }

#else

  PauseNoSafepointVerifier(NoSafepointVerifier * nsv)

    : PauseNoGCVerifier(nsv) {}

  ~PauseNoSafepointVerifier() {}

#endif

};

// A SkipGCALot object is used to elide the usual effect of gc-a-lot

// over a section of execution by a thread. Currently, it's used only to

// prevent re-entrant calls to GC.

class SkipGCALot : public StackObj {

  private:

   bool _saved;

   Thread* _t;

  public:

#ifdef ASSERT

    SkipGCALot(Thread* t) : _t(t) {

      _saved = _t->skip_gcalot();

      _t->set_skip_gcalot(true);

    }

    ~SkipGCALot() {

      assert(_t->skip_gcalot(), "Save-restore protocol invariant");

      _t->set_skip_gcalot(_saved);

    }

#else

    SkipGCALot(Thread* t) { }

    ~SkipGCALot() { }

#endif

};

// JRT_LEAF currently can be called from either _thread_in_Java or

// _thread_in_native mode. In _thread_in_native, it is ok

// for another thread to trigger GC. The rest of the JRT_LEAF

// rules apply.

class JRTLeafVerifier : public NoSafepointVerifier {

  static bool should_verify_GC();

 public:

#ifdef ASSERT

  JRTLeafVerifier();

  ~JRTLeafVerifier();

#else

  JRTLeafVerifier() {}

  ~JRTLeafVerifier() {}

#endif

};

// A NoAllocVerifier object can be placed in methods where one assumes that

// no allocation will occur. The destructor will verify this property

// unless the constructor is called with argument false (not activated).

//

// The check will only be done in debug mode and if activated.

// Note: this only makes sense at safepoints (otherwise, other threads may

// allocate concurrently.)

class NoAllocVerifier : public StackObj {

 private:

  bool  _activated;

 public:

#ifdef ASSERT

  NoAllocVerifier(bool activated = true) {

    _activated = activated;

    if (_activated) Thread::current()->_allow_allocation_count++;

  }

  ~NoAllocVerifier() {

    if (_activated) Thread::current()->_allow_allocation_count--;

  }

#else

  NoAllocVerifier(bool activated = true) {}

  ~NoAllocVerifier() {}

#endif

};

#endif // SHARE_VM_GC_SHARED_GCLOCKER_HPP