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

[DJ Delorie <dj@redhat.com>]

The resulting test has two calls to pthread_barrier_wait() in the
subthread but three calls in main().  These will quickly get out of
sync.


Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org> writes:
> -   - C11 atomics instead of plain access.
> +   - Use pthread_barrier instead of atomic and futexes.

Is this true relative to the original testcase?  Still, merely a
comment, so OK.

> -#include <stdatomic.h>
> -
>  #include <asm/ldt.h>
> -#include <linux/futex.h>

Ok.

> +#include <support/xsignal.h>

Ok.

> -static int
> -futex (int *uaddr, int futex_op, int val, void *timeout, int *uaddr2,
> -	int val3)
> -{
> -  return syscall (SYS_futex, uaddr, futex_op, val, timeout, uaddr2, val3);
> -}

We remove all calls to futex, so no longer need this.  Ok.

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

Ok.

> -/* Possible values of futex:
> -   0: thread is idle.
> -   1: thread armed.
> -   2: thread should clear LDT entry 0.
> -   3: thread should exit.  */
> -static atomic_uint ftx;

Ok.

> +static pthread_barrier_t barrier;

New, ok.

>  static void *
>  threadproc (void *ctx)
>  {
> -  while (1)
> +  for (int i = 0; i < 5; i++)

This matches the loop in main.  Ok.  In the future, a #define loop limit
would be appropriate, to prevent these getting out of sync.  Or a
comment that it has to match main().

>      {
> -      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);

First barrier, ok.

>        /* clear LDT entry 0.  */
>        const struct user_desc desc = { 0 };
>        xmodify_ldt (1, &desc, sizeof (desc));

Leave the stuff the thread is actually testing ;-)

> -      /* 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);

Second barrier, ok.

>      }
> +
> +  return NULL;

Ok, moved from above.

>  }
>  
>  
> @@ -180,6 +162,8 @@ do_test (void)
>    /* Some kernels send SIGBUS instead.  */
>    xsethandler (SIGBUS, sigsegv_handler, 0);
>  
> +  xpthread_barrier_init (&barrier, NULL, 2);

Initialize; must have two callers.  Ok - main and thread both call.

>    thread = xpthread_create (0, threadproc, 0);
>  
>    asm volatile ("mov %%ss, %0" : "=rm" (orig_ss));
> @@ -190,8 +174,7 @@ do_test (void)
>  	continue;
>  
>        /* Make sure the thread is ready after the last test. */
> -      while (atomic_load (&ftx) != 0)
> -	;
> +      xpthread_barrier_wait (&barrier);

First barrier, ok.

>        struct user_desc desc = {
>  	.entry_number       = 0,
> @@ -207,28 +190,21 @@ do_test (void)
>  
>        xmodify_ldt (0x11, &desc, sizeof (desc));
>  
> -      /* Arm the thread.  */
> -      ftx = 1;
> -      futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
> +      xpthread_barrier_wait (&barrier);

Second barrier, ok.
  
>        asm volatile ("mov %0, %%ss" : : "r" (0x7));
>  
> -      /* Fire up thread modify_ldt call.  */
> -      atomic_store (&ftx, 2);
> -
> -      while (atomic_load (&ftx) != 0)
> -	;
> +      xpthread_barrier_wait (&barrier);

Third barrier?  This puts main and the thread out of sync.

>        /* On success, modify_ldt will segfault us synchronously and we will
>  	 escape via siglongjmp.  */
>        support_record_failure ();
>      }
>  
> -  atomic_store (&ftx, 100);
> -  futex ((int*) &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
> -

Ok.

>    xpthread_join (thread);
>  
> +  xpthread_barrier_destroy (&barrier);
> +

Ok.