folly/MicroLock.cpp

   1 /*
   2  * Copyright 2017 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 #include <folly/portability/Asm.h>
  21
  22 namespace folly {
  23
  24 void MicroLockCore::lockSlowPath(uint32_t oldWord,
  25                                  detail::Futex<>* wordPtr,
  26                                  uint32_t slotHeldBit,
  27                                  unsigned maxSpins,
  28                                  unsigned maxYields) {
  29   uint32_t newWord;
  30   unsigned spins = 0;
  31   uint32_t slotWaitBit = slotHeldBit << 1;
  32
  33 retry:
  34   if ((oldWord & slotHeldBit) != 0) {
  35     ++spins;
  36     if (spins > maxSpins + maxYields) {
  37       // Somebody appears to have the lock.  Block waiting for the
  38       // holder to unlock the lock.  We set heldbit(slot) so that the
  39       // lock holder knows to FUTEX_WAKE us.
  40       newWord = oldWord | slotWaitBit;
  41       if (newWord != oldWord) {
  42         if (!wordPtr->compare_exchange_weak(oldWord,
  43                                             newWord,
  44                                             std::memory_order_relaxed,
  45                                             std::memory_order_relaxed)) {
  46           goto retry;
  47         }
  48       }
  49       (void)wordPtr->futexWait(newWord, slotHeldBit);
  50     } else if (spins > maxSpins) {
  51       // sched_yield(), but more portable
  52       std::this_thread::yield();
  53     } else {
  54       folly::asm_volatile_pause();
  55     }
  56     oldWord = wordPtr->load(std::memory_order_relaxed);
  57     goto retry;
  58   }
  59
  60   newWord = oldWord | slotHeldBit;
  61   if (!wordPtr->compare_exchange_weak(oldWord,
  62                                       newWord,
  63                                       std::memory_order_acquire,
  64                                       std::memory_order_relaxed)) {
  65     goto retry;
  66   }
  67 }
  68 } // namespace folly