注意指针修饰符的准确含义

首先从一块儿多线程无锁算法的事故提及,如下是一个无锁栈的实现测试,但在开-O2以上优化的状况下它却没法正常工做:

#include "lf_stack.h"
#include "kn_list.h"
#include "kn_time.h"
#include "kn_thread.h"
#include "kn_atomic.h"

typedef struct lockfree_stack
{
    volatile kn_list_node *head;
}lockfree_stack,*lockfree_stack_t;

static  void lfstack_push(lockfree_stack_t ls,kn_list_node *n)
{
    for( ; ;){  
        kn_list_node *lhead = ls->head;
        n->next = lhead;
        if(COMPARE_AND_SWAP(&ls->head,lhead,n))//if head unchange,set n to be the new head
            break;
    }
}

static  kn_list_node* lfstack_pop(lockfree_stack_t ls)
{
    for( ; ;){  
        kn_list_node *lhead = ls->head;
        if(!lhead) return NULL;
        kn_list_node *next = lhead->next;               
        if(COMPARE_AND_SWAP(&ls->head,lhead,next))
        {
            lhead->next = NULL;
            return lhead;
        }
    }
}


volatile int count = 0;
atomic_32_t c1 = 0;
atomic_32_t c2 = 0;

struct element{
    kn_list_node node;
    int value;
};

struct element *element_pool1;
struct element *element_pool2;

lockfree_stack lf_stack;

void *producer1(void *arg)
{
    printf("producer1\n");
    int i;
    while(1){
        for(i = 0; i < 10000000; ++i){
            struct element *ele =  &element_pool1[i];
            ATOMIC_INCREASE(&c1);
            lfstack_push(&lf_stack,(kn_list_node*)ele);
        }
        while(c1 > 0){
            FENCE();
            kn_sleepms(0);
        }
    }
    printf("producer1 end\n");
        return NULL;
}

void *producer2(void *arg)
{
    printf("producer2\n");
    int i;
    while(1){
        for(i = 0; i < 10000000; ++i){
            struct element *ele =  &element_pool2[i];
            ATOMIC_INCREASE(&c2);   
            lfstack_push(&lf_stack,(kn_list_node*)ele);

        }
        while(c2 > 0){
            FENCE();
            kn_sleepms(0);
        }
    }
    return NULL;
}


void *consumer(void *arg)
{
    printf("consumer\n");
    volatile struct element *ele;
    uint32_t tick = 0;
    while(1){
        if((ele = (struct element*)lfstack_pop(&lf_stack))){
            if(count == 0){
                 tick = kn_systemms();
            }
            if(++count  == 5000000) {
                uint32_t now = kn_systemms();
                            uint32_t elapse = (uint32_t)(now-tick);
                printf("pop %d/ms\n",count/elapse*1000);
                tick = now;
                count = 0;              
            }
            if(ele->value == 1)
                ATOMIC_DECREASE(&c1);
            else if(ele->value == 2)
                ATOMIC_DECREASE(&c2);
            else
                printf("%d\n",ele->value);  
        }
    }
    return NULL;
}

int main(){
    element_pool1 = calloc(10000000,sizeof(*element_pool1));
    element_pool2 = calloc(10000000,sizeof(*element_pool2));
    int i;
    for(i = 0; i < 10000000; ++i) element_pool1[i].value = 1;
    for(i = 0; i < 10000000; ++i) element_pool2[i].value = 2;       
    lf_stack.head = NULL;
    kn_thread_t t1 = kn_create_thread(THREAD_JOINABLE);
    kn_thread_t t2 = kn_create_thread(THREAD_JOINABLE); 
    kn_thread_t t3 = kn_create_thread(THREAD_JOINABLE); 
    kn_thread_start_run(t1,producer1,NULL);
    kn_thread_start_run(t2,producer2,NULL); 
    kn_thread_start_run(t3,consumer,NULL);      
    getchar();
    return 0;
}

表现就是consumer执行必定次数的pop以后死活也没法再弹出元素.不知道各位看官看出问题在哪没有.

当问题再次出现之后,我用调试器上去中断,consumer线程,断点正好在这行if(!lhead) return NULL;,lhead为NULL,我回到上一层栈查看实际上lockfree_stack.head字段并非空,当我想查看lhead的地址时,显示没法查看寄存器地址.

那么问题就明确了,编译器把lhead存放在了寄存器,致使没法发现head实际已经被改变.那么问题来了,我明明将head标记为volatile的呀.

但是再仔细看看,volatile kn_list_node *head;修饰符在指针以前,意思是指向的是volatile变量,而我实际要的是,一个指针它自己是volatile的.ok,作相应的调整后kn_list_node * volatile head;,问题解决.

总之，对指针修饰符关键的一点就是,在*以前,修饰的是指向的目标.而在*以后才是修饰指针自己.