https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66146
--- Comment #47 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:6ee24638ed0ad51e568c799bacf149ba9bd7628b commit r11-7688-g6ee24638ed0ad51e568c799bacf149ba9bd7628b Author: Jonathan Wakely <jwak...@redhat.com> Date: Fri Mar 12 11:47:20 2021 +0000 libstdc++: Revert to old std::call_once implementation [PR 99341] The new std::call_once implementation is not backwards compatible, contrary to my intention. Because std::once_flag::_M_active() doesn't write glibc's "fork generation" into the pthread_once_t object, it's possible for glibc and libstdc++ to run two active executions concurrently. This violates the primary invariant of the feature! This patch reverts std::once_flag and std::call_once to the old implementation that uses pthread_once. This means PR 66146 is a problem again, but glibc has been changed to solve that. A new API similar to pthread_once but supporting failure and resetting the pthread_once_t will be proposed for inclusion in glibc and other C libraries. This change doesn't simply revert r11-4691 because I want to retain the new implementation for non-ghtreads targets (which didn't previously support std::call_once at all, so there's no backwards compatibility concern). This also leaves the new std::call_once::_M_activate() and std::call_once::_M_finish(bool) symbols present in libstdc++.so.6 so that code already compiled against GCC 11 can still use them. Those symbols will be removed in a subsequent commit (which distros can choose to temporarily revert if needed). libstdc++-v3/ChangeLog: PR libstdc++/99341 * include/std/mutex [_GLIBCXX_HAVE_LINUX_FUTEX] (once_flag): Revert to pthread_once_t implementation. [_GLIBCXX_HAVE_LINUX_FUTEX] (call_once): Likewise. * src/c++11/mutex.cc [_GLIBCXX_HAVE_LINUX_FUTEX] (struct __once_flag_compat): New type matching the reverted implementation of once_flag using futexes. (once_flag::_M_activate): Remove, replace with ... (_ZNSt9once_flag11_M_activateEv): ... alias symbol. (once_flag::_M_finish): Remove, replace with ... (_ZNSt9once_flag9_M_finishEb): ... alias symbol. * testsuite/30_threads/call_once/66146.cc: Removed.
