Re: [PATCH v2] i386: Use pthread_barrier for synchronization on tst-bz21269

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 22/02/23 22:08, DJ Delorie wrote:
> Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:
> 
>> -   - C11 atomics instead of plain access.
>> +   - Use pthread_barrier instead of atomic and futexes.
> 
> Ok.
> 
>> -#include <stdatomic.h>
>> -
>>  #include <asm/ldt.h>
>> -#include <linux/futex.h>
> 
> Ok.
> 
>> +#include <support/xsignal.h>
>> +
>> +#define NITER 5
> 
> 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);
>> -}
>> -
> 
> 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;
>> +static pthread_barrier_t barrier;
> 
> Ok.
> 
>> -  while (1)
>> +  for (int i = 0; i < NITER; i++)
> 
> Matches the loop in main, 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);
> 
> First barrier, ok.  Both threads start here
> 
>>        /* clear LDT entry 0.  */
>>        const struct user_desc desc = { 0 };
>>        xmodify_ldt (1, &desc, sizeof (desc));
> 
> While we're doing this, main thread is also modifying its ldt?
> 
>> -      /* 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.  After this, the main thread sets SS...
> 
> I think the barriers are still in the wrong locations.
> 
> the original code did this (wrt main):
> 
> * wait for thread to be idle
> * xmodify_ldt()
> * set thread to "armed"
> * set SS to something "funny"
> * tell thread to run (=2)
> - thread calls xmodify_ltd and goes idle (=0)
> * wait for thread idle
> 
> Then, it sets SS *before* the thread clears the LDT.
> 
> So I think the barriers need to be placed like this:
> 
>   -main-  -thread-
> 
> for (;;) {
>   xmodify_ldt (set it)  -
>   set SS       -
>   -- barrier --
>   -         xmodify_ldt (clear it)
>   -- barrier --
>   fail        -
> }
> 

Ok I thint it works as well.  I will update the patch.