in_commit_epoch_guard.hpp

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/*
 * Copyright (c) 2014-2015, Hewlett-Packard Development Company, LP.
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the Free
 * Software Foundation; either version 2 of the License, or (at your option)
 * any later version.
 *
 * This program 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 for
 * more details. You should have received a copy of the GNU General Public
 * License along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 *
 * HP designates this particular file as subject to the "Classpath" exception
 * as provided by HP in the LICENSE.txt file that accompanied this code.
 */
#ifndef FOEDUS_XCT_IN_COMMIT_EPOCH_GUARD_HPP_
#define FOEDUS_XCT_IN_COMMIT_EPOCH_GUARD_HPP_

#include "foedus/assert_nd.hpp"
#include "foedus/epoch.hpp"
#include "foedus/assorted/atomic_fences.hpp"

namespace foedus {
namespace xct {

struct InCommitEpochGuard {
  InCommitEpochGuard(Epoch* in_commit_epoch_address, Epoch conservative_epoch)
    : in_commit_epoch_address_(in_commit_epoch_address) {
    ASSERT_ND(*in_commit_epoch_address_ == INVALID_EPOCH);
    *in_commit_epoch_address_ = conservative_epoch;
    // We assume the caller of this constructer puts a release or ack_rel fence right after this.
    // So, we don't bother putting the fence here.
    // assorted::memory_fence_release();
  }
  ~InCommitEpochGuard() {
    // prohibit reordering any other change BEFORE the update to in_commit_epoch_.
    // This is to satisfy the first requirement:
    // ("When this returns 0, this transaction will not publish any more log without getting
    // recent epoch").
    // Without this fence, chime can potentially miss the log that has been just published
    // with the old epoch.
    assorted::memory_fence_release();
    *in_commit_epoch_address_ = INVALID_EPOCH;
    // We can also call another memory_order_release here to immediately publish it,
    // but it's anyway rare. The spinning chime will eventually get the update, so no need.
    // In non-TSO architecture, this also saves some overhead in critical path.
  }
  Epoch* const in_commit_epoch_address_;
};
}  // namespace xct
}  // namespace foedus
#endif  // FOEDUS_XCT_IN_COMMIT_EPOCH_GUARD_HPP_