shenandoahEvacOOMHandler.cpp

/*
 * Copyright (c) 2018, Red Hat, Inc. All rights reserved.
 *
 * 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.
 *
 */

#include "precompiled.hpp"

#include "gc_implementation/shenandoah/shenandoahHeap.inline.hpp"
#include "gc_implementation/shenandoah/shenandoahUtils.hpp"
#include "gc_implementation/shenandoah/shenandoahEvacOOMHandler.hpp"

const jint ShenandoahEvacOOMHandler::OOM_MARKER_MASK = 0x80000000;

ShenandoahEvacOOMHandler::ShenandoahEvacOOMHandler() :
  _threads_in_evac(0) {
}

void ShenandoahEvacOOMHandler::wait_for_no_evac_threads() {
  while ((OrderAccess::load_acquire(&_threads_in_evac) & ~OOM_MARKER_MASK) != 0) {
    os::naked_short_sleep(1);
  }
  // At this point we are sure that no threads can evacuate anything. Raise
  // the thread-local oom_during_evac flag to indicate that any attempt
  // to evacuate should simply return the forwarding pointer instead (which is safe now).
  Thread::current()->set_oom_during_evac(true);
}

void ShenandoahEvacOOMHandler::enter_evacuation() {
  jint threads_in_evac = OrderAccess::load_acquire(&_threads_in_evac);

  assert(!Thread::current()->is_evac_allowed(), "sanity");
  assert(!Thread::current()->is_oom_during_evac(), "TL oom-during-evac must not be set");

  if ((threads_in_evac & OOM_MARKER_MASK) != 0) {
    wait_for_no_evac_threads();
    return;
  }

  while (true) {
    jint other = Atomic::cmpxchg(threads_in_evac + 1, &_threads_in_evac, threads_in_evac);
    if (other == threads_in_evac) {
      // Success: caller may safely enter evacuation
      DEBUG_ONLY(Thread::current()->set_evac_allowed(true));
      return;
    } else {
      // Failure:
      //  - if offender has OOM_MARKER_MASK, then loop until no more threads in evac
      //  - otherwise re-try CAS
      if ((other & OOM_MARKER_MASK) != 0) {
        wait_for_no_evac_threads();
        return;
      }
      threads_in_evac = other;
    }
  }
}

void ShenandoahEvacOOMHandler::leave_evacuation() {
  if (!Thread::current()->is_oom_during_evac()) {
    assert((OrderAccess::load_acquire(&_threads_in_evac) & ~OOM_MARKER_MASK) > 0, "sanity");
    // NOTE: It's ok to simply decrement, even with mask set, because unmasked value is positive.
    Atomic::dec(&_threads_in_evac);
  } else {
    // If we get here, the current thread has already gone through the
    // OOM-during-evac protocol and has thus either never entered or successfully left
    // the evacuation region. Simply flip its TL oom-during-evac flag back off.
    Thread::current()->set_oom_during_evac(false);
  }
  DEBUG_ONLY(Thread::current()->set_evac_allowed(false));
  assert(!Thread::current()->is_oom_during_evac(), "TL oom-during-evac must be turned off");
}

void ShenandoahEvacOOMHandler::handle_out_of_memory_during_evacuation() {
  assert(Thread::current()->is_evac_allowed(), "sanity");
  assert(!Thread::current()->is_oom_during_evac(), "TL oom-during-evac must not be set");

  jint threads_in_evac = OrderAccess::load_acquire(&_threads_in_evac);
  while (true) {
    jint other = Atomic::cmpxchg((threads_in_evac - 1) | OOM_MARKER_MASK,
                                  &_threads_in_evac, threads_in_evac);
    if (other == threads_in_evac) {
      // Success: wait for other threads to get out of the protocol and return.
      wait_for_no_evac_threads();
      return;
    } else {
      // Failure: try again with updated new value.
      threads_in_evac = other;
    }
  }
}

void ShenandoahEvacOOMHandler::clear() {
  assert(ShenandoahSafepoint::is_at_shenandoah_safepoint(), "must be at a safepoint");
  assert((OrderAccess::load_acquire(&_threads_in_evac) & ~OOM_MARKER_MASK) == 0, "sanity");
  OrderAccess::release_store_fence(&_threads_in_evac, 0);
}

ShenandoahEvacOOMScope::ShenandoahEvacOOMScope() {
  ShenandoahHeap::heap()->enter_evacuation();
}

ShenandoahEvacOOMScope::~ShenandoahEvacOOMScope() {
  ShenandoahHeap::heap()->leave_evacuation();
}