使用命名信号量的死锁

Question

我正在尝试我的第一个运行使用共享内存和命名信号量来同步对它的访问。

我的程序有 3 个进程 - Parent 一个和两个 child，都必须使用相同的进程共享内存。为了在它们之间同步，我使用了命名的 sempahore。它们共享的资源是一个 3 整数数组，当 array[0] - 退出标志位置 时，child 进程在它们操作之前读取它以确定是否 parent 进程想要退出。 array[1], array[2] - 用于与 parent process 通信，每个进程在自己的数组单元格中放置一条消息，并且 parent进程读取它，并在响应中放置一个 ACK 消息。

我正在尝试获得我的代码的基本工作流程 - 获取所有必要的资源，让 parent 休眠 3 秒，然后启动 exit_procedure。

我的问题是，当到达 exit_procedure 时，主进程永远阻塞在 sem_wait() 操作 - apparently 死锁。我试图找出问题所在，但似乎无法确定。我是处理同步的新手 - 在此代码之前我只同步了线程。

更新：我已经切换到使用 POSIX 内存映射，现在我遇到了同样的死锁问题。 process_api.h 中的相关方法无法获得锁，它们只会永远阻塞。我不知道我做错了什么。有人可以帮忙吗？

我的代码：

主文件：

int *shmem_p;       //!< Shared variable to be used across different proccesses
int shmem_fd;       //!< Shared memory id
sem_t *sem_p;       //!< Sempahore for syncronizing access to shared memory

volatile sig_atomic_t done;         //!< An atomic flag to signal this process threads they are done
volatile sig_atomic_t signal_rcvd;  //!< Indication to exit procedure if a signal caused termination

/**
 * @brief Exit procedure to be called when program is done
 */
static void exit_procedure()
{
    block_all_signals();            /* Block all signals - we're already existing */

    if(signal_rcvd == SIGTERM) {    /* SIGTERM is manually raised by us when a thread terminates, thus not handled in signal handler */
        write(STDERR_FILENO, "Error occured - thread terminated\n", 33);
    }

    if( !signal_rcvd ) {            /* We got here normally, or by thread termination - set done flag */
        done = true;
    }

    /* Free all relevant resources */
    sem_unlink("/shmemory");
    sem_close(sem_p);

    munmap(shmem_p, TOTAL_PROC_NUM*sizeof(int));
    shm_unlink("/shmemory");

    sem_p = NULL;
    shmem_p = NULL;
}

static void signal_handler(int sig_num) {
    switch(sig_num) {
    case SIGCHLD:
        write(STDERR_FILENO, "Error occured - Child process terminated\n", 43);
        break;

    case SIGALRM:
        write(STDOUT_FILENO, "Successfully finished sim\n", 28);
        break;

    default:
        fprintf(stderr, "Error - Signal %s has been raised", strsignal(sig_num));
        fflush(stderr);
        break;
    }

    done = true;
    signal_rcvd = true;
}

static status_t init_procedure()
{
    done = false;
    signal_rcvd = false;
    size_t size = TOTAL_PROC_NUM*sizeof(int);

    /* Initialize shared memory to be used as an exit flag to be used by all processes */

    shmem_fd = shm_open("/shmemory", O_CREAT | O_TRUNC | O_RDWR, 0644);
    if(shmem_fd < 0) {
        error_and_exit("shm_open() failed, err = ", errno);
    }

    if(ftruncate(shmem_fd, size)) {
        shm_unlink("/shmemory");
        error_and_exit("ftruncate() failed, err = ", errno);
    }

    shmem_p = (int *)mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, shmem_fd, 0);
    if(shmem_p == MAP_FAILED) {
        shm_unlink("/shmemory");
        error_and_exit("mmap() failed, err = ", errno);
    }

    close(shmem_fd);    /* No longer needed */
    memset(shmem_p, 0, size);

    /* Initialize a named sempahore for the procceses shared memory */
    sem_p = sem_open("/shsemaphore", O_CREAT | O_RDWR, 0644, 1);
    if(SEM_FAILED == sem_p) {
        error("sem_open() failed, err = ", errno);
        munmap(shmem_p, size);
        shm_unlink("/shmemory");
    }

    /* Initialize memory access invokers processes */
    if(processes_init() != SUCCESS) {
        error("init_processes() failed\n", ERR);
        munmap(shmem_p, size);
        shm_unlink("/shmemory");
        sem_close(sem_p);
        return FAILURE;
    }

    /* Handle Signals - Ignore SIGINT, SIGQUIT, handle SIGCHLD & SIGALRM */

    struct sigaction sig_handler;
    sig_handler.sa_flags = 0;

    if(sigfillset(&sig_handler.sa_mask)) {  /* Mask all other signals inside the handler */
        error("sigemptyset() failed, err = ", errno);
        exit_procedure();
        return FAILURE;
    }

    sig_handler.sa_handler = signal_handler;
    if(sigaction(SIGCHLD, &sig_handler, NULL) || sigaction(SIGALRM, &sig_handler, NULL)) {  /* Set the signal handler for SIGCHLD & SIGALRM */
        error("sigaction() failed, err = ", errno);
        exit_procedure();
        return FAILURE;
    }

    sig_handler.sa_handler = SIG_IGN;
    if(sigaction(SIGINT, &sig_handler, NULL) || sigaction(SIGQUIT, &sig_handler, NULL)) {   /* Ignore ctrl+c and ctrl+z */
        error("sigaction() failed, err = ", errno);
        exit_procedure();
        return FAILURE;
    }

    return SUCCESS;
}

int main(int argc, char *argv[])
{
    if(argc != 1) {
        fprintf(stderr, "usage: %s (no arguments allowed)\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    if(SUCCESS != init_procedure()) {
        error_and_exit("init_procedure() failed\n", ERR);
    }

    sleep(5);
    exit_procedure();

    return EXIT_SUCCESS;
}

进程处理程序：

#define WR_RATE (0.8)               //!< Writing probabilty when invoking memory access
#define WR_RATE_INT (WR_RATE*10)    //!< WR_RATE as an int value between 1 and 10
#define INTER_MEM_ACCS_T (100000)   //!< Waiting time between memory accesses

static pid_t child_pids[CHILD_PROC_NUM];

int process_cnt;                    //!< Determines the index of the process, for child processes
extern sem_t *sem_p;

static bool is_ack_received(int *mem_p, off_t size)
{
    bool ack;

    /*********************************************************/
    /**                 Critical Section start              **/
    if((sem_wait(sem_p) != 0) && (errno != EINTR)) {
        munmap(mem_p, size);
        shm_unlink("/shmemory");
        error_and_Exit("sem_wait() failed, err = ", errno);
    }

    ack = (mem_p[process_cnt] == MSG_ACK);

    if(ack) {// TODO - Remove
        fprintf(stdout, "Process %d received ACK\n", process_cnt);
        fflush(stdout);
    }

    if((sem_post(sem_p) != 0) && (errno != EINTR)) {
        munmap(mem_p, size);
        shm_unlink("/shmemory");
        error_and_Exit("sem_post() failed, err = ", errno);
    }
    /**                 Critical Section end                **/
    /*********************************************************/

    return ack;
}

static void invoke_memory_access(int *mem_p, off_t size)
{
    msg_e send_msg = MSG_READ;
    if(rand_range(1, 10) <= WR_RATE_INT) {  /* Write Memory */
        send_msg = MSG_WRITE;
    }

    /*********************************************************/
    /**                 Critical Section start              **/
    if((sem_wait(sem_p) != 0) && (errno != EINTR)) {  
        munmap(mem_p, size);
        shm_unlink("/shmemory");
        error_and_Exit("sem_wait() failed, err = ", errno);
    }

    mem_p[process_cnt] = send_msg;
    fprintf(stdout, "Process %d sent MSG_%d in mem_address: %p\n", process_cnt, send_msg, &mem_p[process_cnt]); // TODO - Remove
    fflush(stdout);

    if((sem_post(sem_p) != 0) && (errno != EINTR)) {
        munmap(mem_p, size);
        shm_unlink("/shmemory");
        error_and_Exit("sem_post() failed, err = ", errno);
    }
    /**                 Critical Section end                **/
    /*********************************************************/
}

static void main_loop()
{
    int shmem_fd = shm_open("/shmemory", O_RDWR, 0);
    if(shmem_fd < 0) {
        error_and_Exit("shm_open() failed, err = ", errno);
    }

    struct stat mem_stat;
    fstat(shmem_fd, &mem_stat);

    int *child_memory_p = (int *)mmap(NULL, mem_stat.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, shmem_fd, 0);
    if(child_memory_p == MAP_FAILED) {
        shm_unlink("/shmemory");
        error_and_Exit("mmap() failed, err = ", errno);
    }

    close(shmem_fd);    /* No longer needed */

    bool first_run = true;
    bool ack = false;
    const struct timespec ns_wait = {.tv_sec = 0, .tv_nsec = INTER_MEM_ACCS_T};

    while(child_memory_p[0] != MSG_EXIT) {
        if( !first_run ) {                  /* Not the first run, check for ack */
            ack = is_ack_received(child_memory_p, mem_stat.st_size);
        }

        nanosleep(&ns_wait, NULL);

        if( !first_run && !ack ) {  /* No ack received for latest call, nothing to be done */
            continue;
        }

        invoke_memory_access(child_memory_p, mem_stat.st_size);

        if(first_run) {             /* First run is over.. */
            first_run = false;
        }
    }

    fprintf(stdout, "PROCCESS %d EXIT!\n", process_cnt); // TODO Remove this
    fflush(stdout);

    munmap(child_memory_p, mem_stat.st_size);
    shm_unlink("/shmemory");

    child_memory_p = NULL;

    _Exit(EXIT_SUCCESS);
}

status_t processes_init()
{
    pid_t pid;
    process_cnt = 1;    /* Will be used for child processes to determine their order creation */
    int i;
    for(i = 0; i < CHILD_PROC_NUM; ++i) {
        pid = fork();

        if(ERR == pid) {
            error("fork() failed, err = ", errno);
            return FAILURE;
        } else if(pid != 0) {           /* Parent process */
            child_pids[i] = pid;
            process_cnt++;
        } else {                        /* Child process */
            block_all_signals();        /* Only main process responsible for indicate exit to its child*/
            main_loop();
        }
    }

    return SUCCESS;
}

void processes_deinit(int **mem_p)
{
    (*mem_p)[0] = MSG_EXIT;
    fprintf(stdout, "EXIT wrriten to address %p\n", *mem_p);

    /* Wait for all child processes to terminate */
    int i;
    write(STDOUT_FILENO, "Waiting for children to exit\n", 29); // TODO Remove this
    for(i = 0; i < CHILD_PROC_NUM; ++i) {
        if((ERR == waitpid(child_pids[i], NULL, 0)) && (ECHILD != errno)) {
            error("waitpid() failed, err = ", errno);
        }
    }

    fprintf(stdout, "PROCCESS DEINIT DONE!\n"); // TODO Remove this
    fflush(stdout);
}

谁能解释一下我做错了什么？

我试过了：

从主进程传递 sem_t 指针作为 *sem_t **semaphore_p* 到 processes_init 方法，并让每个 child 使用指向信号量的真实指针（即使 child 将指针复制到 COW 机制上，他仍将使用实际地址谢谢
在 进程处理程序中将 sem_t 指针声明为外部指针
打开每个 child 进程（在 main_loop 方法中）一个 "copy" 的命名信号量使用 sem_open("/shsemaphore", O_RDWR)

None 其中有效。伙计们，我要疯了，请帮助我:(

Answer 1

函数init_procedure()中打开信号量后，有一个函数调用

sem_unlink(shared_sem_name);

始终执行，创建后立即销毁信号量。您希望在错误处理块中使用这一行。

Answer 2

找到解决方案：

在主文件中创建命名信号量时，权限设置为 0644，给进程组只读权限。

更改为以下内容后：

sem_open(semaphore_name, O_CREAT, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP, 1)

问题好像解决了！显然，如果在没有 read\write 信号量权限的情况下调用 sem_wait （这发生在子进程中 - 他们仅使用具有 READ 权限的信号量）行为是未定义的

使用命名信号量的死锁

Deadlock using named semaphore

c

deadlock

semaphore

shared-memory

posix-api