What happens if two threads call fork() simultaneously

Question

I have a process with multiple threads. I have registered prepare function and parent handler using __register_atfork(blocksigprof,restoresigprof,NULL,NULL); function. Now let us assume that two threads call fork at the same time. And I have a counter increment in blocksigprof and counter decrement in restoresigprof.

Considering above scenario, will the blocksigprof and restoresigprof be called in pair always? Is there any locking mechanism which inherently done in __register_atfork.

#define NUM_THREADS 8
static int go=0;
static int exec = 1;
static int ev_od = 0;

static void *
test_thread (void *arg) {
    int j;
    pid_t c, d;
    while(!go) // Wait, so that all threads are here.
        continue;
    // All will fork, hopefully at same time because of go signal wait.
    while(exec) {
        c = fork();
        if (c < 0) {
            printf("SANJAY: fork() failed.\n");
            exit(1);
        } else if (c == 0) { // Child
            exit(0);
        }
        else { // parent
            d = waitpid(c, NULL, 0); 
        }
    }   
    return NULL;
}

extern int __register_atfork(void (*)(void),void (*)(void),void (*)(void),void *); 
static sigset_t s_new;
static sigset_t s_old;
static int count = 0;
static void blocksigprof(void){
    count++;
#ifdef SYS_gettid
    pid_t tid = syscall(SYS_gettid);
    if (tid % 2) {
        printf("sleep.\n");
        usleep(1);
    }
#else
#error "SYS_gettid unavailable on this system"
#endif
    printf("Pre-fork. Count should be one. %d\n", count);
}
static void restoresigprof(void){
    printf("Post-fork. Count should be one. %d\n", count);
    count--;
}
int
main () {
    pthread_t t[NUM_THREADS];
    void *ptr;
    long size = 500 * 1024 * 1024;
    int i, m;
    volatile int result = 0;
    int g_iters = 100;

    (void) __register_atfork(blocksigprof,restoresigprof,NULL,NULL);
    // Increase size, so fork takes time.
    printf("SANJAY: Increasing process size.\n");
    ptr = malloc(size);
    memset(ptr, 0,  size);
    ptr = malloc(size);
    memset(ptr, 0,  size);
    ptr = malloc(size);
    memset(ptr, 0,  size);
    ptr = malloc(size);
    memset(ptr, 0,  size);
    ptr = malloc(size);
    memset(ptr, 0,  size);
    ptr = malloc(size);
    memset(ptr, 0,  size);
    ptr = malloc(size);
    memset(ptr, 0,  size);
    ptr = malloc(size);
    memset(ptr, 0,  size);
    ptr = malloc(size);
    memset(ptr, 0,  size);
    // Create threads.
    for (i = 0; i < NUM_THREADS; ++i) {
        pthread_create(&t[i], NULL, test_thread, NULL);
    }

    printf("SANJAY: Killing time.\n");
    // Kill time, so that all threads are at same place post it, waiting for go. 100M cycles.
    for (m = 0; m < 1000000; ++m)
        for (i = 0; i < g_iters; ++i )
            result ^= i;
    // Give all threads go at same time.
    printf("SANJAY: Let threads execute.\n");
    go = 1;
    usleep(10000000); // Wait for 10 sec.
    exec = 0;
    // Wait for all threads to finish.
    for (i = 0; i < NUM_THREADS; ++i) {
        pthread_join(t[i], NULL);
    }
    printf("SANJAY: Done.\n");

    return 0;
}

Answer 1

pthread_atfork specification doesn't require its implementation to serialize calls to prepare and parent handlers, so a safe assumption is that there is no syncronization.

glibc implementation does lock an internal mutex that prevents multiple threads from entering the handlers in parallel. However, that is an implementation detail. The comments in the code say that such an implementation is not POSIX-compliant because POSIX requires pthread_atfork to be async-signal-safe, and using a mutex there makes it not async-signal-safe.

To make your code robust, I recommend using atomics or a mutex to protect your shared state from race condition.