Re: [PATCH v3] i386: Use pthread_barrier for synchronization on tst-bz21269

[DJ Delorie <dj@redhat.com>]

Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:

>  static void *
>  threadproc (void *ctx)
>  {
> -  while (1)
> +  for (int i = 0; i < NITER; i++)

Ok.

> -      futex ((int *) &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
> -      while (atomic_load (&ftx) != 2)
> -	{
> -	  if (atomic_load (&ftx) >= 3)
> -	    return NULL;
> -	}
> +      xpthread_barrier_wait (&barrier);

- parent up to this point has set up the LDT
block 1 - thread waits for parent;

>        /* clear LDT entry 0.  */
>        const struct user_desc desc = { 0 };
>        xmodify_ldt (1, &desc, sizeof (desc));
>  
> -      /* If ftx == 2, set it to zero,  If ftx == 100, quit.  */
> -      if (atomic_fetch_add (&ftx, -2) != 2)
> -	return NULL;
> +      /* Wait for 'ss' set in main thread.  */
> +      xpthread_barrier_wait (&barrier);

block 2 - child sets ldt, parent does nothing

- post, parent sets SS.  This still isn't doing what the original test
case was doing.

The original code did this:


Parent changes LDT and sets SS
- force a task switch, which should segfault
  - sigaction's handler either works, or fails  <-- important part
- child resets LDT

Do you have a version of libc newer than the patch in BZ#21269 that you
can test the modified test against, to make sure it still detects the
failing case?

Thinking about it, I suspect this change shouldn't be done:

> -  TEST_VERIFY_EXIT (sigaction (sig, &sa, 0) == 0);
> +  xsigaction (sig, &sa, 0);

Although they do the same things, since it's sigaction we're actually
testing here, hiding it in an xfunction isn't appropriate.