1/*
2 * Copyright 2016-present Facebook, Inc.
3 *
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
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
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.
15 */
16
17#include <folly/MicroLock.h>
18#include <thread>
19
20#include <folly/portability/Asm.h>
21
22namespace folly {
23
24void MicroLockCore::lockSlowPath(
25 uint32_t oldWord,
26 detail::Futex<>* wordPtr,
27 uint32_t slotHeldBit,
28 unsigned maxSpins,
29 unsigned maxYields) {
30 uint32_t newWord;
31 unsigned spins = 0;
32 uint32_t slotWaitBit = slotHeldBit << 1;
33 uint32_t needWaitBit = 0;
34
35retry:
36 if ((oldWord & slotHeldBit) != 0) {
37 ++spins;
38 if (spins > maxSpins + maxYields) {
39 // Somebody appears to have the lock. Block waiting for the
40 // holder to unlock the lock. We set heldbit(slot) so that the
41 // lock holder knows to FUTEX_WAKE us.
42 newWord = oldWord | slotWaitBit;
43 if (newWord != oldWord) {
44 if (!wordPtr->compare_exchange_weak(
45 oldWord,
46 newWord,
47 std::memory_order_relaxed,
48 std::memory_order_relaxed)) {
49 goto retry;
50 }
51 }
52 detail::futexWait(wordPtr, newWord, slotHeldBit);
53 needWaitBit = slotWaitBit;
54 } else if (spins > maxSpins) {
55 // sched_yield(), but more portable
56 std::this_thread::yield();
57 } else {
58 folly::asm_volatile_pause();
59 }
60 oldWord = wordPtr->load(std::memory_order_relaxed);
61 goto retry;
62 }
63
64 newWord = oldWord | slotHeldBit | needWaitBit;
65 if (!wordPtr->compare_exchange_weak(
66 oldWord,
67 newWord,
68 std::memory_order_acquire,
69 std::memory_order_relaxed)) {
70 goto retry;
71 }
72}
73} // namespace folly
74