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

[Alexandre Oliva <oliva@adacore.com>]

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)
+    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
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