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

[Jonathan Wakely]

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).



>
> -- >8 --
>
> As noted in https://github.com/llvm/llvm-project/issues/62623 there are
> no tsan interceptors for the new POSIX-1:202x APIs added by
> https://austingroupbugs.net/view.php?id=1216 so tsan gives false
> positive warnings.
>
> Disable the uses of the new APIs when tsan is active. This changes the
> semantics of those functions, because it can change which clock is used
> for the wait. This means those functions might be affected by system
> clock adjustments when tsan is used, when they would not be affected
> otherwise.
>
> libstdc++-v3/ChangeLog:
>
>         * acinclude.m4 (GLIBCXX_CHECK_PTHREAD_COND_CLOCKWAIT): Define
>         _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT to depend on _GLIBCXX_TSAN.
>         (GLIBCXX_CHECK_PTHREAD_MUTEX_CLOCKLOCK): Likewise for
>         _GLIBCXX_USE_PTHREAD_MUTEX_CLOCKLOCK.
>         (GLIBCXX_CHECK_PTHREAD_RWLOCK_CLOCKLOCK): Likewise for
>         _GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK.
>         * configure: Regenerate.
> ---
>  libstdc++-v3/acinclude.m4 | 6 +++---
>  libstdc++-v3/configure    | 6 +++---
>  2 files changed, 6 insertions(+), 6 deletions(-)
>
> 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 $glibcxx_cv_PTHREAD_COND_CLOCKWAIT = yes; then
> -    AC_DEFINE(_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT, 1, [Define if
> pthread_cond_clockwait is available in <pthread.h>.])
> +    AC_DEFINE(_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT, (_GLIBCXX_TSAN==0),
> [Define if pthread_cond_clockwait is available in <pthread.h>.])
>    fi
>
>    CXXFLAGS="$ac_save_CXXFLAGS"
> @@ -4314,7 +4314,7 @@ AC_DEFUN([GLIBCXX_CHECK_PTHREAD_MUTEX_CLOCKLOCK], [
>        [glibcxx_cv_PTHREAD_MUTEX_CLOCKLOCK=no])
>    ])
>    if test $glibcxx_cv_PTHREAD_MUTEX_CLOCKLOCK = yes; then
> -    AC_DEFINE(_GLIBCXX_USE_PTHREAD_MUTEX_CLOCKLOCK, 1, [Define if
> pthread_mutex_clocklock is available in <pthread.h>.])
> +    AC_DEFINE(_GLIBCXX_USE_PTHREAD_MUTEX_CLOCKLOCK, (_GLIBCXX_TSAN==0),
> [Define if pthread_mutex_clocklock is available in <pthread.h>.])
>    fi
>
>    CXXFLAGS="$ac_save_CXXFLAGS"
> @@ -4346,7 +4346,7 @@ AC_DEFUN([GLIBCXX_CHECK_PTHREAD_RWLOCK_CLOCKLOCK], [
>        [glibcxx_cv_PTHREAD_RWLOCK_CLOCKLOCK=no])
>    ])
>    if test $glibcxx_cv_PTHREAD_RWLOCK_CLOCKLOCK = yes; then
> -    AC_DEFINE(_GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK, 1, [Define if
> pthread_rwlock_clockrdlock and pthread_rwlock_clockwrlock are available in
> <pthread.h>.])
> +    AC_DEFINE(_GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK, (_GLIBCXX_TSAN==0),
> [Define if pthread_rwlock_clockrdlock and pthread_rwlock_clockwrlock are
> available in <pthread.h>.])
>    fi
>
>    CXXFLAGS="$ac_save_CXXFLAGS"
>
>