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

[Thomas Huth]

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting the bug state to "Incomplete" now.

If the bug has already been fixed in the latest upstream version of QEMU,
then please close this ticket as "Fix released".

If it is not fixed yet and you think that this bug report here is still
valid, then you have two options:

1) If you already have an account on gitlab.com, please open a new ticket
for this problem in our new tracker here:

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

and then close this ticket here on Launchpad (or let it expire auto-
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2) If you don't have an account on gitlab.com and don't intend to get
one, but still would like to keep this ticket opened, then please switch
the state back to "New" within the next 60 days (otherwise it will get
closed as "Expired"). We will then eventually migrate the ticket auto-
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Thank you and sorry for the inconvenience.


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

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

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

Status in QEMU:
  Incomplete

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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