On Feb 17, 2023, Alexandre Oliva <ol...@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)
+ 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>
