Re: [PATCH] [libstdc++] ensure mutex_pool survives _Safe_sequence_base
On 17/02/23 09:01, Alexandre Oliva via Libstdc++ wrote:
On Feb 17, 2023, Alexandre Oliva 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];
}
}
Re: [PATCH] [libstdc++] ensure mutex_pool survives _Safe_sequence_base
On Fri, 17 Feb 2023, 08:02 Alexandre Oliva via Libstdc++, <
libstd...@gcc.gnu.org> wrote:
> On Feb 17, 2023, Alexandre Oliva 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 hackerhttps://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
>
Re: [PATCH] [libstdc++] ensure mutex_pool survives _Safe_sequence_base
On Feb 17, 2023, Alexandre Oliva 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 hackerhttps://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
[PATCH] [libstdc++] ensure mutex_pool survives _Safe_sequence_base
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.
This patch fixes this problem by ensuring the mutex pool completes
construction before any _Safe_sequence_base-containing object, so that
the mutex pool survives them all.
Regstrapped on x86_64-linux-gnu.
Tested on arm-vxworks7 (gcc-12) and arm-eabi (trunk). Ok to install?
for libstdc++-v3/ChangeLog
* include/debug/safe_base.h (_Safe_sequence_base): Ensure
the mutex pool survives *this.
---
libstdc++-v3/include/debug/safe_base.h | 10 +-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/libstdc++-v3/include/debug/safe_base.h
b/libstdc++-v3/include/debug/safe_base.h
index 1dfa9f68b65b5..d4ba404cdac6e 100644
--- a/libstdc++-v3/include/debug/safe_base.h
+++ b/libstdc++-v3/include/debug/safe_base.h
@@ -203,7 +203,15 @@ namespace __gnu_debug
// Initialize with a version number of 1 and no iterators
_Safe_sequence_base() _GLIBCXX_NOEXCEPT
: _M_iterators(0), _M_const_iterators(0), _M_version(1)
-{ }
+{
+ // Make sure the mutex_pool machinery is initialized before any
+ // full object containing a _Safe_sequence_base completes
+ // construction, so that any local static mutexes in the mutex
+ // pool won't be destructed before our destructor runs;
+ // _M_detach_all could fail otherwise, on targets whose mutexes
+ // stop working after being destroyed.
+ (void)this->_M_get_mutex();
+}
#if __cplusplus >= 201103L
_Safe_sequence_base(const _Safe_sequence_base&) noexcept
--
Alexandre Oliva, happy hackerhttps://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
