1 /*
2 * Copyright (c) Facebook, Inc. and its affiliates.
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
19 #include <thread>
20
21 #include <folly/portability/Asm.h>
22
23 namespace folly {
24
lockSlowPath(uint32_t oldWord,detail::Futex<> * wordPtr,unsigned baseShift,unsigned maxSpins,unsigned maxYields)25 uint8_t MicroLockCore::lockSlowPath(
26 uint32_t oldWord,
27 detail::Futex<>* wordPtr,
28 unsigned baseShift,
29 unsigned maxSpins,
30 unsigned maxYields) noexcept {
31 uint32_t newWord;
32 unsigned spins = 0;
33 uint32_t heldBit = 1 << baseShift;
34 uint32_t waitBit = heldBit << 1;
35 uint32_t needWaitBit = 0;
36
37 retry:
38 if ((oldWord & heldBit) != 0) {
39 ++spins;
40 if (spins > maxSpins + maxYields) {
41 // Somebody appears to have the lock. Block waiting for the
42 // holder to unlock the lock. We set heldbit(slot) so that the
43 // lock holder knows to FUTEX_WAKE us.
44 newWord = oldWord | waitBit;
45 if (newWord != oldWord) {
46 if (!wordPtr->compare_exchange_weak(
47 oldWord,
48 newWord,
49 std::memory_order_relaxed,
50 std::memory_order_relaxed)) {
51 goto retry;
52 }
53 }
54 detail::futexWait(wordPtr, newWord, heldBit);
55 needWaitBit = waitBit;
56 } else if (spins > maxSpins) {
57 // sched_yield(), but more portable
58 std::this_thread::yield();
59 } else {
60 folly::asm_volatile_pause();
61 }
62 oldWord = wordPtr->load(std::memory_order_relaxed);
63 goto retry;
64 }
65
66 newWord = oldWord | heldBit | needWaitBit;
67 if (!wordPtr->compare_exchange_weak(
68 oldWord,
69 newWord,
70 std::memory_order_acquire,
71 std::memory_order_relaxed)) {
72 goto retry;
73 }
74 return decodeDataFromWord(newWord, baseShift);
75 }
76 } // namespace folly
77