multithreading - 什么时候应该在多线程编程中使用 'lock'？

Question

我什么时候应该在多线程编程中使用“锁定”？只是锁定每个线程将要修改的区域或锁定每个线程可以访问的区域即使它不会被修改？

struct share_data {
    /* share data */
    thread_id;
}

thread 1 will exceute main() function:
   Initial share data. /* need lock */

   join all thread(share_data.thread_id, &status)   /* access share data thread_id, lock or not? */
   exit.

other threads will:
   access share_data, /* lock or not? */
   modify share_data, /* lock */
   exit.

感谢您的关注，如果您有更多时间，有关真实代码的更多细节:

/* 
the number of threads will be input by user. Structure "tdata" and "tlist" stores 
information of each thread, including: "tid" - thread id which is gaven by 1st argument 
of pthread_create(), "torder" which is the order of calling pthread_create() for each 
thead, "status" stores work status of each thread. I allocate memory for "tlist" 
dynamically. Now, we assume that the number of threads is NUM_THREADS(5). I do not use 
the 4th argument to pass data to each thread, I use global variable "tdata", "tlist" as 
shared data to them. "mutex" variable is used to make sure those threads share data safely,
but it seems not works correctly.

I wanna each thread can get "torder", and maybe modify the status before calling 
pthread_exit().
*/
#include <pthread.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

/* #define NUM_THREADS 5 */
struct tdata {
        pthread_t tid;
        int torder;
        int status;
        struct tdata *next;
};
typedef struct tdata tdata_t;
struct tdatalist {
        tdata_t *head;
        tdata_t *tail;
}tlist;

pthread_mutex_t mutex;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
void *tfunc() {
        tdata_t *p, *q;
        unsigned long int tid;
        int torder;

        p = tlist.head;
        q = NULL;
        pthread_mutex_lock(&mutex);

        tid = pthread_self();

        while (p->tid!=tid && p->next!=NULL) {
                q = p;
                p = p->next;
        }
        tid = p->tid;
        torder = p->torder;
        /* p->status = 0; */
        pthread_mutex_unlock(&mutex);
        /* printf ("I am thread %lu, myorder %d, thread_exit.\n", tid, torder); */
        printf ("I am thread %0x, myorder %d, thread_exit.\n", tid, torder);

        pthread_exit((void *)torder);
}

int main (int argc, char *argv[]) {
/*        pthread_t thread[NUM_THREADS]; */

        pthread_attr_t attr;
        int t;
        tdata_t *tdata_p;
        int num_threads;
        printf ("Input number of threads:");
        scanf ("%d", &num_threads);
        printf ("\n");

        printf ("Main thread id: %08x\n", pthread_self());

        pthread_mutex_init(&mutex, NULL);
        pthread_attr_init(&attr);
        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

        tlist.head=tlist.tail=NULL;     /* create and initial tlist */
        for (t=0; t<num_threads; t++) {

                pthread_mutex_lock(&mutex);
                tdata_p = (tdata_t *) malloc (sizeof (tdata_t));
                pthread_create (&tdata_p->tid, &attr, tfunc, NULL);
                /* tdata_p->tid = thread[t]; */
                tdata_p->torder = t;
                tdata_p->status = 1;          /* for 0,finished the work.  for 1,not*/
                tdata_p->next = NULL;

                if(tlist.head == NULL) {
                        tlist.head = tlist.tail = tdata_p;
                }
                else {
                        tlist.tail->next = tdata_p;
                        tlist.tail = tdata_p;
                }
                pthread_mutex_unlock(&mutex);

        }

        /*  Join child threads */
        pthread_attr_destroy(&attr);
        pthread_mutex_lock (&mutex);
        tdata_t *p;
        tdata_t *q;
        void *status;
        p = tlist.head;
        while (p != NULL) {
                q = p->next;
                pthread_join(p->tid, &status);
                p = q;
        }

        pthread_mutex_unlock (&mutex);

        pthread_mutex_destroy(&mutex);
        /* delete the list */
        p = tlist.head;
        while (p != NULL) {
                q = p->next;
                free (p);
                p = q;
        }
        tlist.head = tlist.tail = NULL;

        printf ("Main exit.\n");
        pthread_exit(NULL);

        return 0;
}

Answer 1

任何时候您要读取或写入数据，都需要锁定它。这可以防止数据尝试读取尚未完成写入的数据。

另一种说法是线程或进程之间共享的任何数据都应该在更改或读取之前锁定。