2 * Copyright 2016 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
17 #include "AsymmetricMemoryBarrier.h"
19 #include <folly/Exception.h>
20 #include <folly/Indestructible.h>
21 #include <folly/portability/SysMembarrier.h>
22 #include <folly/portability/SysMman.h>
29 struct DummyPageCreator {
36 kIsLinux && !detail::sysMembarrierAvailable() ? create() : nullptr;
41 static void* create() {
42 auto ptr = mmap(nullptr, 1, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
43 checkUnixError(reinterpret_cast<uintptr_t>(ptr), "mmap");
45 // Lock the memory so it can't get paged out. If it gets paged out, changing
46 // its protection won't accomplish anything.
47 auto r = mlock(ptr, 1);
48 checkUnixError(r, "mlock");
54 // Make sure dummy page is always initialized before shutdown
55 DummyPageCreator dummyPageCreator;
57 void mprotectMembarrier() {
58 auto dummyPage = dummyPageCreator.get();
60 // This function is required to be safe to call on shutdown,
61 // so we must leak the mutex.
62 static Indestructible<std::mutex> mprotectMutex;
63 std::lock_guard<std::mutex> lg(*mprotectMutex);
66 r = mprotect(dummyPage, 1, PROT_READ | PROT_WRITE);
67 checkUnixError(r, "mprotect");
69 r = mprotect(dummyPage, 1, PROT_READ);
70 checkUnixError(r, "mprotect");
74 void asymmetricHeavyBarrier() {
76 static const bool useSysMembarrier = detail::sysMembarrierAvailable();
77 if (useSysMembarrier) {
78 auto r = detail::sysMembarrier();
79 checkUnixError(r, "membarrier");
84 std::atomic_thread_fence(std::memory_order_seq_cst);