77 lines
2.2 KiB
C++
77 lines
2.2 KiB
C++
/*
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include <folly/MicroLock.h>
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#include <thread>
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#include <folly/portability/Asm.h>
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namespace folly {
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uint8_t MicroLockCore::lockSlowPath(
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uint32_t oldWord,
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detail::Futex<>* wordPtr,
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unsigned baseShift,
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unsigned maxSpins,
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unsigned maxYields) noexcept {
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uint32_t newWord;
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unsigned spins = 0;
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uint32_t heldBit = 1 << baseShift;
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uint32_t waitBit = heldBit << 1;
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uint32_t needWaitBit = 0;
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retry:
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if ((oldWord & heldBit) != 0) {
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++spins;
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if (spins > maxSpins + maxYields) {
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// Somebody appears to have the lock. Block waiting for the
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// holder to unlock the lock. We set heldbit(slot) so that the
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// lock holder knows to FUTEX_WAKE us.
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newWord = oldWord | waitBit;
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if (newWord != oldWord) {
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if (!wordPtr->compare_exchange_weak(
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oldWord,
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newWord,
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std::memory_order_relaxed,
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std::memory_order_relaxed)) {
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goto retry;
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}
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}
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detail::futexWait(wordPtr, newWord, heldBit);
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needWaitBit = waitBit;
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} else if (spins > maxSpins) {
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// sched_yield(), but more portable
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std::this_thread::yield();
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} else {
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folly::asm_volatile_pause();
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}
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oldWord = wordPtr->load(std::memory_order_relaxed);
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goto retry;
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}
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newWord = oldWord | heldBit | needWaitBit;
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if (!wordPtr->compare_exchange_weak(
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oldWord,
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newWord,
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std::memory_order_acquire,
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std::memory_order_relaxed)) {
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goto retry;
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}
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return decodeDataFromWord(newWord, baseShift);
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}
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} // namespace folly
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