Java example source code file (gcLocker.hpp)

This example Java source code file (gcLocker.hpp) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

assert, atomic\:\:dec, jrt_leaf_verifier, no_alloc_verifier, no_gc_verifier, no_safepoint_verifier, pause_no_gc_verifier, pause_no_safepoint_verifier, share_vm_memory_gclocker_hpp, skipgcalot, stackobj, target_os_family_solaris, target_os_family_windows, thread\:\:current

The gcLocker.hpp Java example source code

/*
 * Copyright (c) 1997, 2013, 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

#ifndef SHARE_VM_MEMORY_GCLOCKER_HPP
#define SHARE_VM_MEMORY_GCLOCKER_HPP

#include "gc_interface/collectedHeap.hpp"
#include "memory/genCollectedHeap.hpp"
#include "memory/universe.hpp"
#include "oops/oop.hpp"
#include "runtime/thread.inline.hpp"
#ifdef TARGET_OS_FAMILY_linux
# include "os_linux.inline.hpp"
#endif
#ifdef TARGET_OS_FAMILY_solaris
# include "os_solaris.inline.hpp"
#endif
#ifdef TARGET_OS_FAMILY_windows
# include "os_windows.inline.hpp"
#endif
#ifdef TARGET_OS_FAMILY_bsd
# include "os_bsd.inline.hpp"
#endif

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

  static volatile jint _lock_count;      // number of other 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

  // Accessors
  static bool is_jni_active() {
    assert(_needs_gc, "only valid when _needs_gc is set");
    return _jni_lock_count > 0;
  }

  // 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 _lock_count > 0 || _jni_lock_count > 0;
  }

 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() {
#ifdef ASSERT
    assert(_debug_jni_lock_count >= 0, "bad value");
    Atomic::inc(&_debug_jni_lock_count);
#endif
  }
  static void decrement_debug_jni_lock_count() {
#ifdef ASSERT
    assert(_debug_jni_lock_count > 0, "bad value");
    Atomic::dec(&_debug_jni_lock_count);
#endif
  }

  // 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();

  // Non-structured GC locking: currently needed for JNI. Use with care!
  static void lock();
  static void unlock();

  // The following two methods are used for JNI critical regions.
  // If we find that we failed to perform a GC because the GC_locker
  // 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,
  // GC_locker::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 No_GC_Verifier 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 No_GC_Verifier: public StackObj {
 friend class Pause_No_GC_Verifier;

 protected:
  bool _verifygc;
  unsigned int _old_invocations;

 public:
#ifdef ASSERT
  No_GC_Verifier(bool verifygc = true);
  ~No_GC_Verifier();
#else
  No_GC_Verifier(bool verifygc = true) {}
  ~No_GC_Verifier() {}
#endif
};

// A Pause_No_GC_Verifier is used to temporarily pause the behavior
// of a No_GC_Verifier object. If we are not in debug mode or if the
// No_GC_Verifier object has a _verifygc value of false, then there
// is nothing to do.

class Pause_No_GC_Verifier: public StackObj {
 private:
  No_GC_Verifier * _ngcv;

 public:
#ifdef ASSERT
  Pause_No_GC_Verifier(No_GC_Verifier * ngcv);
  ~Pause_No_GC_Verifier();
#else
  Pause_No_GC_Verifier(No_GC_Verifier * ngcv) {}
  ~Pause_No_GC_Verifier() {}
#endif
};


// A No_Safepoint_Verifier 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 No_GC_Verifier
//
class No_Safepoint_Verifier : public No_GC_Verifier {
 friend class Pause_No_Safepoint_Verifier;

 private:
  bool _activated;
  Thread *_thread;
 public:
#ifdef ASSERT
  No_Safepoint_Verifier(bool activated = true, bool verifygc = true ) :
    No_GC_Verifier(verifygc),
    _activated(activated) {
    _thread = Thread::current();
    if (_activated) {
      _thread->_allow_allocation_count++;
      _thread->_allow_safepoint_count++;
    }
  }

  ~No_Safepoint_Verifier() {
    if (_activated) {
      _thread->_allow_allocation_count--;
      _thread->_allow_safepoint_count--;
    }
  }
#else
  No_Safepoint_Verifier(bool activated = true, bool verifygc = true) : No_GC_Verifier(verifygc){}
  ~No_Safepoint_Verifier() {}
#endif
};

// A Pause_No_Safepoint_Verifier is used to temporarily pause the
// behavior of a No_Safepoint_Verifier object. If we are not in debug
// mode then there is nothing to do. If the No_Safepoint_Verifier
// 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 No_GC_Verifier object.

class Pause_No_Safepoint_Verifier : public Pause_No_GC_Verifier {
 private:
  No_Safepoint_Verifier * _nsv;

 public:
#ifdef ASSERT
  Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
    : Pause_No_GC_Verifier(nsv) {

    _nsv = nsv;
    if (_nsv->_activated) {
      _nsv->_thread->_allow_allocation_count--;
      _nsv->_thread->_allow_safepoint_count--;
    }
  }

  ~Pause_No_Safepoint_Verifier() {
    if (_nsv->_activated) {
      _nsv->_thread->_allow_allocation_count++;
      _nsv->_thread->_allow_safepoint_count++;
    }
  }
#else
  Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
    : Pause_No_GC_Verifier(nsv) {}
  ~Pause_No_Safepoint_Verifier() {}
#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 JRT_Leaf_Verifier : public No_Safepoint_Verifier {
  static bool should_verify_GC();
 public:
#ifdef ASSERT
  JRT_Leaf_Verifier();
  ~JRT_Leaf_Verifier();
#else
  JRT_Leaf_Verifier() {}
  ~JRT_Leaf_Verifier() {}
#endif
};

// A No_Alloc_Verifier 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 No_Alloc_Verifier : public StackObj {
 private:
  bool  _activated;

 public:
#ifdef ASSERT
  No_Alloc_Verifier(bool activated = true) {
    _activated = activated;
    if (_activated) Thread::current()->_allow_allocation_count++;
  }

  ~No_Alloc_Verifier() {
    if (_activated) Thread::current()->_allow_allocation_count--;
  }
#else
  No_Alloc_Verifier(bool activated = true) {}
  ~No_Alloc_Verifier() {}
#endif
};

#endif // SHARE_VM_MEMORY_GCLOCKER_HPP