Re: [PATCH] [libstdc++] ensure mutex_pool survives _Safe_sequence_base

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Fri, 17 Feb 2023, 08:02 Alexandre Oliva via Libstdc++, <
libstdc++@gcc.gnu.org> wrote:

> On Feb 17, 2023, Alexandre Oliva <oliva@adacore.com> wrote:
>
> > On vxworks, after destroying the semaphore used to implement a mutex,
> > __gthread_mutex_lock fails and __gnu_cxx::__mutex::lock calls
> > __throw_concurrence_lock_error.  Nothing ensures the mutex_pool
> > mutexes survive init-once objects containing _Safe_sequence_base.  If
> > such an object completes construction before mutex_pool
> > initialization, it will be registered for atexit destruction after the
> > mutex_pool mutexes, so the _M_detach_all() call in the
> > _Safe_sequence_base dtor will use already-destructed mutexes, and
> > basic_string/requirements/citerators_cc fails calling terminate.
>
> Here's an alternative approach, with zero runtime overhead.  Negative
> overhead, if you count the time it would have taken to destruct the
> mutex pool :-) But it fails to destruct them, which is presumably of no
> consequence.
>

Agreed, I was going to suggest we immortalise them like this.



> [libstdc++] do not destruct mutex_pool mutexes
>
> [Copy of the paragraph quoted above omitted here]
>
> This patch fixes this problem by ensuring the mutex pool mutexes are
> constructed on demand, on a statically-allocated buffer, but never
> destructed.
>
> Regstrapped on x86_64-linux-gnu.
> Tested on arm-vxworks7 (gcc-12) and arm-eabi (trunk).  Ok to install?
>


OK, thanks.



> for  libstdc++-v3/ChangeLog
>
>         * src/c++11/shared_ptr.cc (__gnu_internal::get_mutex):
>         Avoid destruction of the mutex pool.
> ---
>  libstdc++-v3/src/c++11/shared_ptr.cc |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/src/c++11/shared_ptr.cc
> b/libstdc++-v3/src/c++11/shared_ptr.cc
> index bc70134359c87..74e879e582896 100644
> --- a/libstdc++-v3/src/c++11/shared_ptr.cc
> +++ b/libstdc++-v3/src/c++11/shared_ptr.cc
> @@ -36,7 +36,11 @@ namespace __gnu_internal _GLIBCXX_VISIBILITY(hidden)
>    {
>      // increase alignment to put each lock on a separate cache line
>      struct alignas(64) M : __gnu_cxx::__mutex { };
> -    static M m[mask + 1];
> +    // Use a static buffer, so that the mutexes are not destructed
> +    // before potential users (or at all)
> +    static __attribute__ ((aligned(__alignof__(M))))
> +      char buffer[(sizeof (M)) * (mask + 1)];
> +    static M *m = new (buffer) M[mask + 1];
>      return m[i];
>    }
>  }
>
> --
> 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>
>