首先从一起多线程无锁算法的事故说起,以下是一个无锁栈的实现测试,但在开-O2以上优化的情况下它却无法正常工作:

#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执行一定次数的pop之后死活也无法再弹出元素.不知道各位看官看出问题在哪没有.

当问题再次出现以后,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字段并不是空,当我想查看lhead的地址时,显示无法查看寄存器地址.

那么问题就明确了,编译器把lhead存放在了寄存器,导致无法发现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.

可是再仔细看看,volatile kn_list_node *head;修饰符在指针之前,意思是指向的是volatile变量,而我实际要的是,一个指针它本身是volatile的.ok,做相应的调整后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*之前,修饰的是指向的目标.而在*之后才是修饰指针本身.