01d412ef36
The user-visible effect of this change is for std::future::wait_until to use CLOCK_MONOTONIC when passed a timeout of std::chrono::steady_clock type. This makes it immune to any changes made to the system clock CLOCK_REALTIME. Add an overload of __atomic_futex_unsigned::_M_load_and_text_until_impl that accepts a std::chrono::steady_clock, and correctly passes this through to __atomic_futex_unsigned_base::_M_futex_wait_until_steady which uses CLOCK_MONOTONIC for the timeout within the futex system call. These functions are mostly just copies of the std::chrono::system_clock versions with small tweaks. Prior to this commit, a std::chrono::steady timeout would be converted via std::chrono::system_clock which risks reducing or increasing the timeout if someone changes CLOCK_REALTIME whilst the wait is happening. (The commit immediately prior to this one increases the window of opportunity for that from a short period during the calculation of a relative timeout, to the entire duration of the wait.) FUTEX_WAIT_BITSET was added in kernel v2.6.25. If futex reports ENOSYS to indicate that this operation is not supported then the code falls back to using clock_gettime(2) to calculate a relative time to wait for. I believe that I've added this functionality in a way that it doesn't break ABI compatibility, but that has made it more verbose and less type safe. I believe that it would be better to maintain the timeout as an instance of the correct clock type all the way down to a single _M_futex_wait_until function with an overload for each clock. The current scheme of separating out the seconds and nanoseconds early risks accidentally calling the wait function for the wrong clock. Unfortunately, doing this would break code that compiled against the old header. libstdc++-v3/ChangeLog: * config/abi/pre/gnu.ver: Update for addition of __atomic_futex_unsigned_base::_M_futex_wait_until_steady. * include/bits/atomic_futex.h (__atomic_futex_unsigned_base): Add comments to clarify that _M_futex_wait_until and _M_load_and_test_until use CLOCK_REALTIME. (__atomic_futex_unsigned_base::_M_futex_wait_until_steady) (__atomic_futex_unsigned_base::_M_load_and_text_until_steady): New member functions that use CLOCK_MONOTONIC. (__atomic_futex_unsigned_base::_M_load_and_test_until_impl) (__atomic_futex_unsigned_base::_M_load_when_equal_until): Add overloads that accept a steady_clock time_point and use the new member functions. * src/c++11/futex.cc: Include headers required for clock_gettime. (futex_clock_monotonic_flag): New constant to tell futex to use CLOCK_MONOTONIC to match existing futex_clock_realtime_flag. (futex_clock_monotonic_unavailable): New global to store the result of trying to use CLOCK_MONOTONIC. (__atomic_futex_unsigned_base::_M_futex_wait_until_steady): Add new variant of _M_futex_wait_until that uses CLOCK_MONOTONIC to support waiting using steady_clock. |
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