From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from rock.gnat.com (rock.gnat.com [IPv6:2620:20:4000:0:a9e:1ff:fe9b:1d1]) by sourceware.org (Postfix) with ESMTPS id 346F33858C78; Fri, 17 Feb 2023 08:02:16 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 346F33858C78 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=adacore.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=adacore.com Received: from localhost (localhost.localdomain [127.0.0.1]) by filtered-rock.gnat.com (Postfix) with ESMTP id EDE6D116B6A; Fri, 17 Feb 2023 03:02:15 -0500 (EST) X-Virus-Scanned: Debian amavisd-new at gnat.com Received: from rock.gnat.com ([127.0.0.1]) by localhost (rock.gnat.com [127.0.0.1]) (amavisd-new, port 10024) with LMTP id QpgEt9dFg-vk; Fri, 17 Feb 2023 03:02:15 -0500 (EST) Received: from free.home (tron.gnat.com [IPv6:2620:20:4000:0:46a8:42ff:fe0e:e294]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by rock.gnat.com (Postfix) with ESMTPS id 86A75116B59; Fri, 17 Feb 2023 03:02:15 -0500 (EST) Received: from livre (livre.home [172.31.160.2]) by free.home (8.15.2/8.15.2) with ESMTPS id 31H81wSg093234 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NOT); Fri, 17 Feb 2023 05:01:58 -0300 From: Alexandre Oliva To: gcc-patches@gcc.gnu.org Cc: libstdc++@gcc.gnu.org Subject: Re: [PATCH] [libstdc++] ensure mutex_pool survives _Safe_sequence_base Organization: Free thinker, does not speak for AdaCore References: Errors-To: aoliva@lxoliva.fsfla.org Date: Fri, 17 Feb 2023 05:01:58 -0300 In-Reply-To: (Alexandre Oliva's message of "Fri, 17 Feb 2023 04:44:11 -0300") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.84 X-Spam-Status: No, score=-12.3 required=5.0 tests=BAYES_00,GIT_PATCH_0,KAM_DMARC_STATUS,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: 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 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