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- /*
- * Copyright 2016-present Facebook, Inc.
- *
- * 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/MicroLock.h>
- #include <thread>
- #include <folly/portability/Asm.h>
- namespace folly {
- void MicroLockCore::lockSlowPath(
- uint32_t oldWord,
- detail::Futex<>* wordPtr,
- uint32_t slotHeldBit,
- unsigned maxSpins,
- unsigned maxYields) {
- uint32_t newWord;
- unsigned spins = 0;
- uint32_t slotWaitBit = slotHeldBit << 1;
- uint32_t needWaitBit = 0;
- retry:
- if ((oldWord & slotHeldBit) != 0) {
- ++spins;
- if (spins > maxSpins + maxYields) {
- // Somebody appears to have the lock. Block waiting for the
- // holder to unlock the lock. We set heldbit(slot) so that the
- // lock holder knows to FUTEX_WAKE us.
- newWord = oldWord | slotWaitBit;
- if (newWord != oldWord) {
- if (!wordPtr->compare_exchange_weak(
- oldWord,
- newWord,
- std::memory_order_relaxed,
- std::memory_order_relaxed)) {
- goto retry;
- }
- }
- detail::futexWait(wordPtr, newWord, slotHeldBit);
- needWaitBit = slotWaitBit;
- } else if (spins > maxSpins) {
- // sched_yield(), but more portable
- std::this_thread::yield();
- } else {
- folly::asm_volatile_pause();
- }
- oldWord = wordPtr->load(std::memory_order_relaxed);
- goto retry;
- }
- newWord = oldWord | slotHeldBit | needWaitBit;
- if (!wordPtr->compare_exchange_weak(
- oldWord,
- newWord,
- std::memory_order_acquire,
- std::memory_order_relaxed)) {
- goto retry;
- }
- }
- } // namespace folly
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