POSIX 计时器在运行几次后挂断

Question

我在程序的主函数中创建了一个 POSIX 计时器。主程序的每个线程都设置定时器，以便在定时器到期时，信号处理程序更新一个变量，该变量唤醒同一进程的下一个线程。

计时器大部分时间工作正常，但并非总是如此。它有时会完成完整的执行，而在其他运行中，它会挂起。可能的原因是什么？我怀疑是和信号传输有关。

代码如下：

#define _GNU_SOURCE
#define _POSIX_C_SOURCE 199309
#include <sched.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <signal.h>
#include <errno.h>
#include <semaphore.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <syscall.h>
#define NUM_THREADS 10

#define CLOCKID CLOCK_REALTIME
#define SIG SIGUSR1
int ret;
timer_t timerid;
struct sigevent sev;
struct itimerspec its;
long long freq_nanosecs;
sigset_t mask;
struct sigaction sa;

sem_t sem[NUM_THREADS];
sem_t mute;

pthread_t tid[NUM_THREADS];
int state = 0;

static void handler(int sig, siginfo_t *si, void *uc)
{
    ret = sem_post(&sem[(state+1)%NUM_THREADS]);
        if (ret)
        {
            printf("Error in Sem Post\n");
        }
        state++;
}

void *threadA(void *data_)
{  
    int i = 0, s,n,value;

    long int loopNum;
    int turn = (intptr_t)data_;
    struct timespec tval_result,tval_result2;

    int sid = syscall(SYS_gettid);
    FILE *fp;
    fp=fopen("ipc.out","a");    
    fprintf(fp,"thread_%d %d\n",turn,sid);  
    fclose(fp); 

    int counter=0;

    while(1)
    {
        ret = sem_wait(&sem[turn]);
        if (ret)
        {
            printf("Error in Sem Post\n");
        }
        //printf("Thread # -> %d\n",turn);

        its.it_value.tv_sec = 0;
        its.it_value.tv_nsec = 14000;
        its.it_interval.tv_sec = 0;
        its.it_interval.tv_nsec = 0;

        ret = timer_settime(timerid, 0, &its, NULL);
        if ( ret < 0 )
            perror("timer_settime");

        // Some heavy work

    counter++;

    if(counter==100)
    break;
    }
    printf("finished %d\n",turn);

}

int main(int argc, char *argv[])
{
    int data = 0;
    int err,i;

    sa.sa_flags = SA_RESTART;
    sa.sa_sigaction = handler;
    sigemptyset(&sa.sa_mask);
    sigaction(SIG, &sa, NULL);

    sev.sigev_notify = SIGEV_SIGNAL;
    sev.sigev_signo = SIG;
    sev.sigev_value.sival_ptr = &timerid;
    ret = timer_create(CLOCKID, &sev, &timerid);
    if ( ret < 0 )
        perror("timer_create");

    sem_init(&sem[0], 0, 1); 
    for ( i = 1; i < NUM_THREADS; ++i)
        {
            sem_init(&sem[i], 0, 0); 
        }

    while(data < NUM_THREADS)
    {
        //create our threads
        err = pthread_create(&tid[data], NULL, threadA, (void *)(intptr_t)data);
        if(err != 0)
            printf("\ncan't create thread :[%s]", strerror(err));

        data++;
    }

    pthread_exit(NULL);
}

据此，这个程序应该打印

finished 0
finished 1
finished 2
finished 3
finished 4
finished 5
finished 6
finished 7
finished 8
finished 9

有时会这样打印，但大多数时候，程序会挂起。

Answer 1

信号处理程序存在竞争条件。一旦 sem_post 被调用，其他线程之一就可以启动运行并且它的计时器可以在当前信号处理程序完成之前触发。这将导致在另一个线程中再次调用信号处理程序。那时 state 还没有被第一个线程递增，因此第二个信号处理程序调用最终将在错误的信号量上调用 sem_post。

解决此问题的一种方法是确保在调用 sem_post:

之前递增 state

static void handler(int sig, siginfo_t *si, void *uc)
{
    state++;
    ret = sem_post(&sem[(state)%NUM_THREADS]);
    if (ret)
    {
        printf("Error in Sem Post\n");
    }
}

请注意，此解决方案仍然存在一个问题。它不能确保 printf 调用的顺序正确。

POSIX 计时器在运行几次后挂断

POSIX timer hangs up after a few runs

c

linux

multithreading

posix

timer