From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id F0C623985802; Sat, 26 Sep 2020 19:41:22 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org F0C623985802 From: "cvs-commit at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug libstdc++/96817] __cxa_guard_acquire unsafe against dynamically loaded pthread Date: Sat, 26 Sep 2020 19:41:22 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: libstdc++ X-Bugzilla-Version: 10.2.0 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: cvs-commit at gcc dot gnu.org X-Bugzilla-Status: NEW X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gcc-bugs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-bugs mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 26 Sep 2020 19:41:23 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96817 --- Comment #17 from CVS Commits --- The master branch has been updated by Jonathan Wakely : https://gcc.gnu.org/g:e6923541fae5081b646f240d54de2a32e17a0382 commit r11-3484-ge6923541fae5081b646f240d54de2a32e17a0382 Author: Jonathan Wakely Date: Sat Sep 26 20:32:36 2020 +0100 libstdc++: Use __libc_single_threaded to optimise atomics [PR 96817] Glibc 2.32 adds a global variable that says whether the process is single-threaded. We can use this to decide whether to elide atomic operations, as a more precise and reliable indicator than __gthread_active_p. This means that guard variables for statics and reference counting in shared_ptr can use less expensive, non-atomic ops even in processes that are linked to libpthread, as long as no threads have been created yet. It also means that we switch to using atomics if libpthread gets loaded later via dlopen (this still isn't supported in general, for other reasons). We can't use __libc_single_threaded to replace __gthread_active_p everywhere. If we replaced the uses of __gthread_active_p in std::mutex then we would elide the pthread_mutex_lock in the code below, but not the pthread_mutex_unlock: std::mutex m; m.lock(); // pthread_mutex_lock std::thread t([]{}); // __libc_single_threaded =3D false t.join(); m.unlock(); // pthread_mutex_unlock We need the lock and unlock to use the same "is threading enabled" predicate, and similarly for init/destroy pairs for mutexes and condition variables, so that we don't try to release resources that were never acquired. There are other places that could use __libc_single_threaded, such as _Sp_locker in src/c++11/shared_ptr.cc and locale init functions, but they can be changed later. libstdc++-v3/ChangeLog: PR libstdc++/96817 * include/ext/atomicity.h (__gnu_cxx::__is_single_threaded()): New function wrapping __libc_single_threaded if available. (__exchange_and_add_dispatch, __atomic_add_dispatch): Use it. * libsupc++/guard.cc (__cxa_guard_acquire, __cxa_guard_abort) (__cxa_guard_release): Likewise. * testsuite/18_support/96817.cc: New test.=