https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78237
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> --- Author: redi Date: Mon Dec 2 16:23:10 2019 New Revision: 278903 URL: https://gcc.gnu.org/viewcvs?rev=278903&root=gcc&view=rev Log: libstdc++: Add full steady_clock support to shared_timed_mutex The pthread_rwlock_clockrdlock and pthread_rwlock_clockwrlock functions were added to glibc in v2.30. They have also been added to Android Bionic. If these functions are available in the C library then they can be used to implement shared_timed_mutex::try_lock_until, shared_timed_mutex::try_lock_for, shared_timed_mutex::try_lock_shared_until and shared_timed_mutex::try_lock_shared_for so that they are no longer unaffected by the system clock being warped. (This is the shared_mutex equivalent of PR libstdc++/78237 for mutex.) If the new functions are available then steady_clock is deemed to be the "best" clock available which means that it is used for the relative try_lock_for calls and absolute try_lock_until calls using steady_clock and user-defined clocks. It's not possible to have _GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK defined without _GLIBCXX_USE_PTHREAD_RWLOCK_T, so the requirement that the clock be the same as condition_variable is maintained. Calls explicitly using system_clock (aka high_resolution_clock) continue to use CLOCK_REALTIME via the old pthread_rwlock_timedrdlock and pthread_rwlock_timedwrlock functions. If the new functions are not available then system_clock is deemed to be the "best" clock available which means that the previous suboptimal behaviour remains. Additionally, the user-defined clock used with shared_timed_mutex::try_lock_for and shared_mutex::try_lock_shared_for may have higher precision than __clock_t. We may need to round the duration up to ensure that the timeout is long enough. (See __timed_mutex_impl::_M_try_lock_for) 2019-12-02 Mike Crowe <m...@mcrowe.com> Add full steady_clock support to shared_timed_mutex * acinclude.m4 (GLIBCXX_CHECK_PTHREAD_RWLOCK_CLOCKLOCK): Define to check for the presence of both pthread_rwlock_clockrdlock and pthread_rwlock_clockwrlock. * config.h.in: Regenerate. * configure.ac: Call GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK. * configure: Regenerate. * include/std/shared_mutex (shared_timed_mutex): Define __clock_t as the best clock to use for relative waits. (shared_timed_mutex::try_lock_for) Round up wait duration if necessary. (shared_timed_mutex::try_lock_shared_for): Likewise. (shared_timed_mutex::try_lock_until): Use existing try_lock_until implementation for system_clock (which matches __clock_t when _GLIBCCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK is not defined). Add new overload for steady_clock that uses pthread_rwlock_clockwrlock if it is available. Simplify overload for non-standard clock to just call try_lock_for with a relative timeout. (shared_timed_mutex::try_lock_shared_until): Likewise. Modified: trunk/libstdc++-v3/ChangeLog trunk/libstdc++-v3/acinclude.m4 trunk/libstdc++-v3/config.h.in trunk/libstdc++-v3/configure trunk/libstdc++-v3/configure.ac trunk/libstdc++-v3/include/std/shared_mutex
