1 /* 2 * Copyright 2021 The Android Open Source Project 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 #pragma once 18 #include <atomic> 19 20 template <typename T> 21 // Single consumer multi producer stack. We can understand the two operations independently to see 22 // why they are without race condition. 23 // 24 // push is responsible for maintaining a linked list stored in mPush, and called from multiple 25 // threads without lock. We can see that if two threads never observe the same value from 26 // mPush.load, it just functions as a normal linked list. In the case where two threads observe the 27 // same value, one of them has to execute the compare_exchange first. The one that doesn't execute 28 // the compare exchange first, will receive false from compare_exchange. previousHead is updated (by 29 // compare_exchange) to the most recent value of mPush, and we try again. It's relatively clear to 30 // see that the process can repeat with an arbitrary number of threads. 31 // 32 // Pop is much simpler. If mPop is empty (as it begins) it atomically exchanges 33 // the entire push list with null. This is safe, since the only other reader (push) 34 // of mPush will retry if it changes in between it's read and atomic compare. We 35 // then store the list and pop one element. 36 // 37 // If we already had something in the pop list we just pop directly. 38 class LocklessStack { 39 public: 40 class Entry { 41 public: 42 T* mValue; 43 std::atomic<Entry*> mNext; Entry(T * value)44 Entry(T* value): mValue(value) {} 45 }; 46 std::atomic<Entry*> mPush = nullptr; 47 std::atomic<Entry*> mPop = nullptr; push(T * value)48 void push(T* value) { 49 Entry* entry = new Entry(value); 50 Entry* previousHead = mPush.load(/*std::memory_order_relaxed*/); 51 do { 52 entry->mNext = previousHead; 53 } while (!mPush.compare_exchange_weak(previousHead, entry)); /*std::memory_order_release*/ 54 } pop()55 T* pop() { 56 Entry* popped = mPop.load(/*std::memory_order_acquire*/); 57 if (popped) { 58 // Single consumer so this is fine 59 mPop.store(popped->mNext/* , std::memory_order_release */); 60 auto value = popped->mValue; 61 delete popped; 62 return value; 63 } else { 64 Entry *grabbedList = mPush.exchange(nullptr/* , std::memory_order_acquire */); 65 if (!grabbedList) return nullptr; 66 mPop.store(grabbedList->mNext/* , std::memory_order_release */); 67 auto value = grabbedList->mValue; 68 delete grabbedList; 69 return value; 70 } 71 } 72 }; 73