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/*
* Copyright (c) Facebook, Inc. and its affiliates.
*
* 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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