[RFC] libstdc++: Do not use pthread_mutex_clocklock with ThreadSanitizer

[Mike Crowe]

On Wednesday 10 May 2023 at 12:31:12 +0100, Jonathan Wakely wrote:
> On Wed, 10 May 2023 at 12:20, Jonathan Wakely via Libstdc++ <
> libstdc++@gcc.gnu.org> wrote:
> 
> > This patch would avoid TSan false positives when using timed waiting
> > functions on mutexes and condvars, but as noted below, it changes the
> > semantics.
> >
> > I'm not sure whether we want this workaround in place until tsan gets
> > fixed.
> >
> > On one hand, there's no guarantee that those functions use the right
> > clock anyway (and they won't do unless a recent-ish glibc is used). But
> > on the other hand, if they normally would use the right clock because
> > you have glibc support, it's not ideal for tsan to cause a different
> > clock to be used.
> >
> 
> But of course, it's not ideal to get false positives from tsan either
> (especially when it looks like a libstdc++ bug, as initially reported to
> me).

I think that this is probably the least-worst option in the short term. As
TSan is distributed with GCC this workaround can be removed as soon as its
TSan implementation gains the necessary interceptors. I shall look into
trying to do that.

However, ...

> > diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
> > index 89e7f5f5f45..e2700b05ec3 100644
> > --- a/libstdc++-v3/acinclude.m4
> > +++ b/libstdc++-v3/acinclude.m4
> > @@ -4284,7 +4284,7 @@ AC_DEFUN([GLIBCXX_CHECK_PTHREAD_COND_CLOCKWAIT], [
> >        [glibcxx_cv_PTHREAD_COND_CLOCKWAIT=no])
> >    ])
> >    if test $glibcxx_cv_PTHREAD_COND_CLOCKWAIT = yes; then
> > -    AC_DEFINE(_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT, 1, [Define if
> > pthread_cond_clockwait is available in <pthread.h>.])
> > +    AC_DEFINE(_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT, (_GLIBCXX_TSAN==0),
> > [Define if pthread_cond_clockwait is available in <pthread.h>.])
> >    fi

TSan does appear to have an interceptor for pthread_cond_clockwait, even if
it lacks the others. Does this mean that this part is unnecessary?

See: https://github.com/google/sanitizers/issues/1259

Thanks.

Mike.