Re: [PATCH 1/3] Use reliable sem_wait interruption in nptl/tst-sem6.

[Torvald Riegel <triegel at redhat dot com>]

On Sat, 2014-12-06 at 14:50 +0100, OndÅej BÃlka wrote:
> On Fri, Dec 05, 2014 at 07:52:57PM +0100, Torvald Riegel wrote:
> >    alarm (1);
> >  
> >    int res = sem_wait (&s);
> > -  if (res == 0)
> > -    {
> > -      puts ("wait succeeded");
> > -      return 1;
> > -    }
> > -  if (res != -1 || errno != EINTR)
> > +  /* We accept all allowed behavior: Implementations that return EINTR and
> > +     those that rely on correct interruption through signals to use sem_post
> > +     in the signal handler.  */
> > +  if (res != 0 && !(res == -1 && errno == EINTR))
> >      {
> > -      puts ("wait didn't fail with EINTR");
> > +      puts ("wait neiter succeeded nor failed with EINTR");
> >        return 1;
> >      }
> >  
> 
> That does change test logic as it originally failed when wait succeeded.

Yes, but why do you think that this is inconsistent?  The previous test
didn't add a token in the signal handler, so if wait succeeded, then the
test should fail.

However, the correct way to interrupt the semaphore with a signal is to
add a token.  My patch does that.  Second, if we do not want to make
timing assumptions (which the existing test would do if we add a token
to the semaphore in the signal handler), then we need to accept that the
(first) signal handler execution might happen before sem_wait actually
executes.  Therefore, we must not fail in this case.

We have to correctly interrupt with signals because as the futex
documentation stands (allowing return of EINTR on spurious wake-ups),
there's no way to implement the specific behavior of the existing
implementation (which assumes EINTR is returned *only* on signals).

IOW, the existing test does white-box testing with timing assumptions;
with this patch, we do make a slightly different black-box test with no
timing assumptions.