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
  19 namespace folly {
  20
  21 void MicroLockCore::lockSlowPath(uint32_t oldWord,
  22                                  detail::Futex<>* wordPtr,
  23                                  uint32_t slotHeldBit,
  24                                  unsigned maxSpins,
  25                                  unsigned maxYields) {
  26   unsigned newWord;
  27   unsigned spins = 0;
  28   uint32_t slotWaitBit = slotHeldBit << 1;
  29
  30 retry:
  31   if ((oldWord & slotHeldBit) != 0) {
  32     ++spins;
  33     if (spins > maxSpins + maxYields) {
  34       // Somebody appears to have the lock.  Block waiting for the
  35       // holder to unlock the lock.  We set heldbit(slot) so that the
  36       // lock holder knows to FUTEX_WAKE us.
  37       newWord = oldWord | slotWaitBit;
  38       if (newWord != oldWord) {
  39         if (!wordPtr->compare_exchange_weak(oldWord,
  40                                             newWord,
  41                                             std::memory_order_relaxed,
  42                                             std::memory_order_relaxed)) {
  43           goto retry;
  44         }
  45       }
  46       (void)wordPtr->futexWait(newWord, slotHeldBit);
  47     } else if (spins > maxSpins) {
  48       sched_yield();
  49     }
  50     oldWord = wordPtr->load(std::memory_order_relaxed);
  51     goto retry;
  52   }
  53
  54   newWord = oldWord | slotHeldBit;
  55   if (!wordPtr->compare_exchange_weak(oldWord,
  56                                       newWord,
  57                                       std::memory_order_relaxed,
  58                                       std::memory_order_relaxed)) {
  59     goto retry;
  60   }
  61
  62   // Locks are traditionally memory barriers, so we emit a full fence
  63   // even though we were happy using relaxed atomics for the
  64   // lock itself.
  65   std::atomic_thread_fence(std::memory_order_seq_cst);
  66 }
  67 }