folly/folly/synchronization/AsymmetricThreadFence.cpp

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <folly/synchronization/AsymmetricThreadFence.h>

#include <mutex>

#include <folly/Exception.h>
#include <folly/Indestructible.h>
#include <folly/portability/SysMembarrier.h>
#include <folly/portability/SysMman.h>
#include <folly/synchronization/RelaxedAtomic.h>

namespace folly {

namespace {

// The intention is to force a memory barrier in every core running any of the
// process's threads. There is not a wide selection of options, but we do have
// one trick: force a TLB shootdown. There are multiple scenarios in which a TLB
// shootdown occurs, two of which are relevant: (1) when a resident page is
// swapped out, and (2) when the protection on a resident page is downgraded.
// We cannot force (1) and we cannot force (2). But we can force at least one of
// the outcomes (1) or (2) to happen!
void mprotectMembarrier() { … }

bool sysMembarrierAvailableCached() { … }

} // namespace

void asymmetric_thread_fence_heavy_fn::impl_(std::memory_order order) noexcept { … }

} // namespace folly