redis源码阅读-atomicvar(原子操作)

¶redis源码阅读-atomicvar(原子操作)

¶atomicvar介绍

• atomicvar是redis封装的原子操作函数。其中包括了三种原子操作方法，gcc提供的內建__atomic和__sync,mutex函数族以及pthread线程库提供的mutex系列函数。
• 原子操作主要是争对多线程下的线程安全。

¶atomicvar.h

/* This file implements atomic counters using __atomic or __sync macros if
    * available, otherwise synchronizing different threads using a mutex.
    *
    * The exported interface is composed of three macros:
    *
    * atomicIncr(var,count) -- Increment the atomic counter
    * atomicGetIncr(var,oldvalue_var,count) -- Get and increment the atomic counter
    * atomicDecr(var,count) -- Decrement the atomic counter
    * atomicGet(var,dstvar) -- Fetch the atomic counter value
    * atomicSet(var,value)  -- Set the atomic counter value
    *
    * The variable 'var' should also have a declared mutex with the same
    * name and the "_mutex" postfix, for instance:
    *
    *  long myvar;
    *  pthread_mutex_t myvar_mutex;
    *  atomicSet(myvar,12345);
    *
    * If atomic primitives are available (tested in config.h) the mutex
    * is not used.
    *
    * Never use return value from the macros, instead use the AtomicGetIncr()
    * if you need to get the current value and increment it atomically, like
    * in the followign example:
    *
    *  long oldvalue;
    *  atomicGetIncr(myvar,oldvalue,1);
    *  doSomethingWith(oldvalue);
    *
    * ----------------------------------------------------------------------------
    *
    * Copyright (c) 2015, Salvatore Sanfilippo <antirez at gmail dot com>
    * All rights reserved.
    *
    * Redistribution and use in source and binary forms, with or without
    * modification, are permitted provided that the following conditions are met:
    *
    *   * Redistributions of source code must retain the above copyright notice,
    *     this list of conditions and the following disclaimer.
    *   * Redistributions in binary form must reproduce the above copyright
    *     notice, this list of conditions and the following disclaimer in the
    *     documentation and/or other materials provided with the distribution.
    *   * Neither the name of Redis nor the names of its contributors may be used
    *     to endorse or promote products derived from this software without
    *     specific prior written permission.
    *
    * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
    * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
    * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
    * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
    * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
    * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
    * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
    * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
    * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
    * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
    * POSSIBILITY OF SUCH DAMAGE.
    */
/***********************************************************************************
    * 实现gcc内置函数(__atomic和__sync),实现线程的原子操作
    * redis定义了__atomic,__sync,mutex三种原子操作方法，最多使用的是__atomic或者__sync(__atomic是用于替代__sync函数的，现在大多数使用__atomic进行原子操作),不能使用时就可以选择mutex(线程互斥量)
    * 参考文档：
    *  官方手册：https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html#g_t_005f_005fatomic-Builtins
    *  gcc的原子操作：https://blog.csdn.net/qq_22660775/article/details/88806716
    *
    * 提供函数：
    *  atomicIncr(var,count) -- 增加原子计数器
    *  atomicGetIncr(var,oldvalue_var,count) -- 获取并增加原子计数器
    *  atomicDecr(var,count) -- 减少原子计数器
    *  atomicGet(var,dstvar) -- 获取原子计数器的值
    *  atomicSet(var,value)  -- 设置原子计数器的值
    ***********************************************************************************/
#include <pthread.h> // 提供操作线程的函数原型
#ifndef __ATOMIC_VAR_H
#define __ATOMIC_VAR_H
/* To test Redis with Helgrind (a Valgrind tool) it is useful to define
    * the following macro, so that __sync macros are used: those can be detected
    * by Helgrind (even if they are less efficient) so that no false positive
    * is reported. */
// #define __ATOMIC_VAR_FORCE_SYNC_MACROS
// Helgrind用于检查线程中出现的竞争问题，在这里被用于检测定义的宏是否有用
// 如果没有定义__ATOMIC_VAR_FORCE_SYNC_MACROS(强制使用__sync)并且定义了__atomic
// 如果没有使用sun,mac(版本在4210057之上的mac例外)系列操作系统，clang编译器
// 满足以上条件就使用__atomic宏进行原子操作
#if !defined(__ATOMIC_VAR_FORCE_SYNC_MACROS) && defined(__ATOMIC_RELAXED) && !defined(__sun) && (!defined(__clang__) || !defined(__APPLE__) || __apple_build_version__ > 4210057)
/* Implementation using __atomic macros. */
// type __atomic_add_fetch (type *ptr, type val, int memorder) 原子加操作(先增加后提交)
// type *ptr 要修改的原子对象指针
// type val 要添加到存储在原子对象中的值的值
// int memorder 此操作的内存同步排序：所有值都是允许的
// 参考：https://m.php.cn/manual/view/34159.html
#define atomicIncr(var,count) __atomic_add_fetch(&var,(count),__ATOMIC_RELAXED)
// 先提交修改，然后增加原子计数器并返回原子计数器
#define atomicGetIncr(var,oldvalue_var,count) do { \
    oldvalue_var = __atomic_fetch_add(&var,(count),__ATOMIC_RELAXED); \
} while(0)
    // 先减再提交
#define atomicDecr(var,count) __atomic_sub_fetch(&var,(count),__ATOMIC_RELAXED)
// 以原子方式加载，并返回原子原子变量的当前值
#define atomicGet(var,dstvar) do { \
    dstvar = __atomic_load_n(&var,__ATOMIC_RELAXED); \
} while(0)
// 将value值存储到原子对象(&var)的值中
#define atomicSet(var,value) __atomic_store_n(&var,value,__ATOMIC_RELAXED)
#define REDIS_ATOMIC_API "atomic-builtin"
#elif defined(HAVE_ATOMIC)
/* Implementation using __sync macros. */
// 与上诉方法差不多，只是底层实现使用__sync来实现
#define atomicIncr(var,count) __sync_add_and_fetch(&var,(count))
#define atomicGetIncr(var,oldvalue_var,count) do { \
    oldvalue_var = __sync_fetch_and_add(&var,(count)); \
} while(0)
#define atomicDecr(var,count) __sync_sub_and_fetch(&var,(count))
#define atomicGet(var,dstvar) do { \
    dstvar = __sync_sub_and_fetch(&var,0); \
} while(0)
#define atomicSet(var,value) do { \
    while(!__sync_bool_compare_and_swap(&var,var,value)); \
} while(0)
#define REDIS_ATOMIC_API "sync-builtin"
#else
/* Implementation using pthread mutex. */
// 使用mutex来实现
// &var ## _mutex : 将传入的原子对象变量值与_mutex连接，使_mutex作为后缀
// int pthread_mutex_lock(pthread_mutex_t *mutex); // 通过线程互斥量来加锁,之后就对数值var进行加减等操作，完了就解锁
#define atomicIncr(var,count) do { \
    pthread_mutex_lock(&var ## _mutex); \
    var += (count); \
    pthread_mutex_unlock(&var ## _mutex); \
} while(0)
#define atomicGetIncr(var,oldvalue_var,count) do { \
    pthread_mutex_lock(&var ## _mutex); \
    oldvalue_var = var; \
    var += (count); \
    pthread_mutex_unlock(&var ## _mutex); \
} while(0)
#define atomicDecr(var,count) do { \
    pthread_mutex_lock(&var ## _mutex); \
    var -= (count); \
    pthread_mutex_unlock(&var ## _mutex); \
} while(0)
#define atomicGet(var,dstvar) do { \
    pthread_mutex_lock(&var ## _mutex); \
    dstvar = var; \
    pthread_mutex_unlock(&var ## _mutex); \
} while(0)
#define atomicSet(var,value) do { \
    pthread_mutex_lock(&var ## _mutex); \
    var = value; \
    pthread_mutex_unlock(&var ## _mutex); \
} while(0)
#define REDIS_ATOMIC_API "pthread-mutex"
#endif
#endif /* __ATOMIC_VAR_H */