首先從一起多線程無鎖算法的事故說起,以下是一個無鎖棧的實現測試,但在開-O2以上優化的情況下它卻無法正常工作:

#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 headbreak;} }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);elseprintf("%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; }

表現就是consumer執行一定次數的pop之后死活也無法再彈出元素.不知道各位看官看出問題在哪沒有.

當問題再次出現以后,我用調試器上去中斷,consumer線程,斷點正好在這行if(!lhead) return NULL;,lhead為NULL,我回到上一層棧查看實際上lockfree_stack.head字段并不是空,當我想查看lhead的地址時,顯示無法查看寄存器地址.

那么問題就明確了,編譯器把lhead存放在了寄存器,導致無法發現head實際已經被改變.那么問題來了,我明明將head標記為volatile的呀.

可是再仔細看看,volatile kn_list_node *head;修飾符在指針之前,意思是指向的是volatile變量,而我實際要的是,一個指針它本身是volatile的.ok,做相應的調整后kn_list_node * volatile head;,問題解決.

總之，對指針修飾符關鍵的一點就是,在*之前,修飾的是指向的目標.而在*之后才是修飾指針本身.

轉載于:https://www.cnblogs.com/sniperHW/p/4190088.html

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