[Bug 1887306] Re: qemu-user deadlocks when forked in a multithreaded process

[Thomas Huth]

This is an automated cleanup. This bug report has been moved to QEMU's
new bug tracker on gitlab.com and thus gets marked as 'expired' now.
Please continue with the discussion here:

 https://gitlab.com/qemu-project/qemu/-/issues/358


** Changed in: qemu
       Status: New => Expired

** Bug watch added: gitlab.com/qemu-project/qemu/-/issues #358
   https://gitlab.com/qemu-project/qemu/-/issues/358

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https://bugs.launchpad.net/bugs/1887306

Title:
  qemu-user deadlocks when forked in a multithreaded process

Status in QEMU:
  Expired

Bug description:
  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.

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