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

[Jonathan Wakely <jwakely@redhat.com>]

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On Fri, 3 Mar 2023 at 09:33, Jonathan Wakely <jwakely@redhat.com> wrote:

>
>
> 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.
>

As Jakub pointed out, it adds a scheduling barrier, but a few cycles when
creating a new thread is probably not even measurable.


>
> 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).
>

I should be clear that I don't think the creative solution is a good idea.
And if we ever support building libstdc++ with LTO it will become visible
that the argument is unused, and we'd need something like the asm
dependency anyway.



>
>
>
>>
>>
>> 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>
>>
>>