Re: [libstdc++] Use __gthread_join in jthread/95989

[Jonathan Wakely <jwakely@redhat.com>]

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On Fri, 3 Mar 2023 at 08:20, Alexandre Oliva via Libstdc++ <
libstdc++@gcc.gnu.org> wrote:

> On Feb 17, 2023, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
> > The proper fix is to ensure the program has a non-weak reference to
> > pthread_join without extra help (or use a recent glibc where it always
> > works).
>
> Indeed!  How about this?  Regstrapped on x86_64-linux-gnu, also tested
> on arm-vx7r2 (gcc-12); verified that strong references are present in
> 95989.o, but not in libstdc++.a or .so.  Ok to install?
>
> (nit: the static local in a ctor of a template class may make for
> multiple copies.  Maybe a non-template always-inline function called by
> all instantiations would be better.)
>

Yeah, that does seem less than ideal.

Jakub previously suggested doing this for PR 61841, which was a similar
problem with pthread_create:

__asm ("" : : "r" (&pthread_create)); would not be optimized away.


That would avoid the multiple copies.

Alternatively we could get really creative and cast the addresses of both
&pthread_create and &pthread_join to uintptr_t and XOR them, and pass that
as the __depend argument (which is never actually dereferenced, it's only
there to create a link-time dependency).



>
>
> link pthread_join from std::thread ctor
>
> Like pthread_create, pthread_join may fail to be statically linked in
> absent strong uses, so add to user code strong references to both when
> std::thread objects are created.
>
>
> for  libstdc++-v3/ChangeLog
>
>         * include/bits/std_thread.h (std::thread ctor): Add strong
>         reference to pthread_join.
>
> ---
>  libstdc++-v3/include/bits/std_thread.h |    1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/libstdc++-v3/include/bits/std_thread.h
> b/libstdc++-v3/include/bits/std_thread.h
> index adbd3928ff783..4bda350fa2c7b 100644
> --- a/libstdc++-v3/include/bits/std_thread.h
> +++ b/libstdc++-v3/include/bits/std_thread.h
> @@ -145,6 +145,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  #ifdef GTHR_ACTIVE_PROXY
>         // Create a reference to pthread_create, not just the gthr weak
> symbol.
>         auto __depend = reinterpret_cast<void(*)()>(&pthread_create);
> +       static auto __attribute__((__used__)) __depend_join =
> &pthread_join;
>  #else
>         auto __depend = nullptr;
>  #endif
>
>
> --
> Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
>    Free Software Activist                       GNU Toolchain Engineer
> Disinformation flourishes because many people care deeply about injustice
> but very few check the facts.  Ask me about <https://stallmansupport.org>
>
>