qemu-user deadlocks when forked in a multithreaded process

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1887306
Reported by 'Alexey Izbyshev' on 2020-07-12 :

The following program (also attached) deadlocks when run under QEMU user
on Linux.

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#define NUM_THREADS 100
#define NUM_FORKS 10

pthread_barrier_t barrier;

void *t(void *arg) {
    for (int i = 0; i < NUM_FORKS; i++) {
        pid_t pid = fork();
        if (pid < 0)
            abort();
        if (!pid)
            _exit(0);
        if (waitpid(pid, NULL, 0) < 0)
            abort();
    }
    //pthread_barrier_wait(&barrier);
    return NULL;
}

int main(void) {
    pthread_barrier_init(&barrier, NULL, NUM_THREADS);
    pthread_t ts[NUM_THREADS];
    for (size_t i = 0; i < NUM_THREADS; i++) {
        if (pthread_create(&ts[i], NULL, t, NULL))
            abort();
    }
    for (size_t i = 0; i < NUM_THREADS; i++) {
        pthread_join(ts[i], NULL);
    }
    printf("Done: %d\n", getpid());
    return 0;
}

To reproduce:
$ gcc test.c -pthread
$ while qemu-x86_64 ./a.out; do :; done

(Be careful, Ctrl-C/SIGINT doesn't kill the deadlocked child).

Larger values of NUM_THREADS/NUM_FORKS lead to more often deadlocks. With
the values above it often deadlocks on the first try on my machine. When
it deadlocks, there is a child qemu process with two threads which is
waited upon by one of the worker threads of the parent.

I tried to avoid the deadlock by serializing fork() with a mutex, but it
didn't help. However, ensuring that no thread exits until all forks are
done (by adding a barrier to t()) does seem to help, at least, the program
above could run for a half an hour until I terminated it.

Tested on QEMU 5.0.0, 4.2.0 and 2.11.1, with x86_64 and AArch64 linux-user
targets.