From: "François Dumont" <frs.dumont@gmail.com>
To: Alexandre Oliva <oliva@adacore.com>, gcc-patches@gcc.gnu.org
Cc: libstdc++@gcc.gnu.org
Subject: Re: [PATCH] [libstdc++] ensure mutex_pool survives _Safe_sequence_base
Date: Fri, 17 Feb 2023 21:33:11 +0100 [thread overview]
Message-ID: <c08a16f5-d17e-a8bb-a8c7-3c77d4b56c72@gmail.com> (raw)
In-Reply-To: <orcz68ra89.fsf@lxoliva.fsfla.org>
On 17/02/23 09:01, Alexandre Oliva via Libstdc++ 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.
>
> [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?
>
> 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)
I guess you meant 'before potential use'
> + static __attribute__ ((aligned(__alignof__(M))))
> + char buffer[(sizeof (M)) * (mask + 1)];
> + static M *m = new (buffer) M[mask + 1];
> return m[i];
> }
> }
>
prev parent reply other threads:[~2023-02-17 20:33 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-17 7:44 Alexandre Oliva
2023-02-17 8:01 ` Alexandre Oliva
2023-02-17 11:03 ` Jonathan Wakely
2023-02-17 20:33 ` François Dumont [this message]
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