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 <folly/MicroLock.h>
20 #include <folly/portability/Asm.h>
24 void MicroLockCore::lockSlowPath(uint32_t oldWord,
25 detail::Futex<>* wordPtr,
31 uint32_t slotWaitBit = slotHeldBit << 1;
34 if ((oldWord & slotHeldBit) != 0) {
36 if (spins > maxSpins + maxYields) {
37 // Somebody appears to have the lock. Block waiting for the
38 // holder to unlock the lock. We set heldbit(slot) so that the
39 // lock holder knows to FUTEX_WAKE us.
40 newWord = oldWord | slotWaitBit;
41 if (newWord != oldWord) {
42 if (!wordPtr->compare_exchange_weak(oldWord,
44 std::memory_order_relaxed,
45 std::memory_order_relaxed)) {
49 (void)wordPtr->futexWait(newWord, slotHeldBit);
50 } else if (spins > maxSpins) {
51 // sched_yield(), but more portable
52 std::this_thread::yield();
56 oldWord = wordPtr->load(std::memory_order_relaxed);
60 newWord = oldWord | slotHeldBit;
61 if (!wordPtr->compare_exchange_weak(oldWord,
63 std::memory_order_acquire,
64 std::memory_order_relaxed)) {