Re: [PATCH v2] libstdc++: Do not use pthread_mutex_clocklock with ThreadSanitizer

[Mike Crowe <mac@mcrowe.com>]

On Thursday 11 May 2023 at 21:52:22 +0100, Jonathan Wakely wrote:
> On Thu, 11 May 2023 at 13:42, Jonathan Wakely <jwakely@redhat.com> wrote:
> 
> >
> >
> > On Thu, 11 May 2023 at 13:19, Mike Crowe <mac@mcrowe.com> wrote:
> >
> >> 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?
> >>
> >
> > Ah good point, thanks. I grepped for clocklock but not clockwait.
> >
> 
> In fact it seems like we don't need to change
> _GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK either, because I don't get any tsan
> warnings for that. It doesn't have interceptors for
> pthread_rwlock_{rd,wr}lock, but it doesn't complain anyway (maybe it's
> simply not instrumenting the rwlock functions at all?!)

It looks like TSan does have interceptors for pthread_rwlock_timedrdlock
etc. I can't explain why this doesn't cause problems when libstdc++ uses
pthread_rwlock_clockrdlock etc.

> So I'm now retesting with this version of the patch, which only touches the
> USE_PTHREAD_LOCKLOCK macro.
> 
> Please take another look, thanks.

> commit 4fc14825c125eece32980df21d09da35e3d5bac6
> Author: Jonathan Wakely <jwakely@redhat.com>
> Date:   Tue May 9 09:30:48 2023
> 
>     libstdc++: Do not use pthread_mutex_clocklock with ThreadSanitizer
>     
>     As noted in https://github.com/llvm/llvm-project/issues/62623 there are
>     no tsan interceptors for some of the new POSIX-1:202x APIs added by
>     https://austingroupbugs.net/view.php?id=1216 so tsan gives false
>     positive warnings for try_lock_for on timed mutexes.
>     
>     Disable the uses of the new pthread_mutex_clocklock API when tsan is
>     active. This changes the semantics of the try_lock_for 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_MUTEX_CLOCKLOCK): Define
>             _GLIBCXX_USE_PTHREAD_MUTEX_CLOCKLOCK in terms of _GLIBCXX_TSAN.
>             * configure: Regenerate.
> 
> diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
> index 89e7f5f5f45..dce3d16aa5c 100644
> --- a/libstdc++-v3/acinclude.m4
> +++ b/libstdc++-v3/acinclude.m4
> @@ -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"

LGTM.

Mike.