folly/MicroLock.cpp

   1 /*
   2  * Copyright 2016 Facebook, Inc.
   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 #include <thread>
  19
  20 namespace folly {
  21
  22 void MicroLockCore::lockSlowPath(uint32_t oldWord,
  23                                  detail::Futex<>* wordPtr,
  24                                  uint32_t slotHeldBit,
  25                                  unsigned maxSpins,
  26                                  unsigned maxYields) {
  27   unsigned newWord;
  28   unsigned spins = 0;
  29   uint32_t slotWaitBit = slotHeldBit << 1;
  30
  31 retry:
  32   if ((oldWord & slotHeldBit) != 0) {
  33     ++spins;
  34     if (spins > maxSpins + maxYields) {
  35       // Somebody appears to have the lock.  Block waiting for the
  36       // holder to unlock the lock.  We set heldbit(slot) so that the
  37       // lock holder knows to FUTEX_WAKE us.
  38       newWord = oldWord | slotWaitBit;
  39       if (newWord != oldWord) {
  40         if (!wordPtr->compare_exchange_weak(oldWord,
  41                                             newWord,
  42                                             std::memory_order_relaxed,
  43                                             std::memory_order_relaxed)) {
  44           goto retry;
  45         }
  46       }
  47       (void)wordPtr->futexWait(newWord, slotHeldBit);
  48     } else if (spins > maxSpins) {
  49       // sched_yield(), but more portable
  50       std::this_thread::yield();
  51     } else {
  52       folly::asm_pause();
  53     }
  54     oldWord = wordPtr->load(std::memory_order_relaxed);
  55     goto retry;
  56   }
  57
  58   newWord = oldWord | slotHeldBit;
  59   if (!wordPtr->compare_exchange_weak(oldWord,
  60                                       newWord,
  61                                       std::memory_order_acquire,
  62                                       std::memory_order_relaxed)) {
  63     goto retry;
  64   }
  65 }
  66 }