On 01/08/18 14:19 +0100, Mike Crowe wrote:
The futex system call supports waiting for an absolute time if FUTEX_WAIT_BITSET is used rather than FUTEX_WAIT. Doing so provides two benefits:1. The call to gettimeofday is not required in order to calculate a relative timeout. 2. If someone changes the system clock during the wait then the futex timeout will correctly expire earlier or later. Currently that only happens if the clock is changed prior to the call to gettimeofday. According to futex(2), support for FUTEX_CLOCK_REALTIME was added in the v2.6.28 Linux kernel and FUTEX_WAIT_BITSET was added in v2.6.25. To ensure that the code still works correctly with earlier kernel versions, an ENOSYS error from futex[1] results in the futex_clock_realtime_unavailable being flag being set. This flag is used to avoid the unnecessary unsupported futex call in the future and to fall back to the previous gettimeofday and relative time implementation. glibc applied an equivalent switch in pthread_cond_timedwait to use FUTEX_CLOCK_REALTIME and FUTEX_WAIT_BITSET rather than FUTEX_WAIT for glibc-2.10 back in 2009. See glibc:cbd8aeb836c8061c23a5e00419e0fb25a34abee7 The futex_clock_realtime_unavailable flag is accessed using std::memory_order_relaxed to stop it becoming a bottleneck. If the first two calls to _M_futex_wait_until happen to happen simultaneously then the only consequence is that both will try to use FUTEX_CLOCK_REALTIME, both risk discovering that it doesn't work and, if so, both set the flag. [1] This is how glibc's nptl-init.c determines whether these flags are supported. --- libstdc++-v3/src/c++11/futex.cc | 34 ++++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) diff --git a/libstdc++-v3/src/c++11/futex.cc b/libstdc++-v3/src/c++11/futex.cc index 278a5a80902..72062a4285e 100644 --- a/libstdc++-v3/src/c++11/futex.cc +++ b/libstdc++-v3/src/c++11/futex.cc @@ -35,8 +35,16 @@ // Constants for the wait/wake futex syscall operations const unsigned futex_wait_op = 0; +const unsigned futex_wait_bitset_op = 9; +const unsigned futex_clock_realtime_flag = 256; +const unsigned futex_bitset_match_any = ~0; const unsigned futex_wake_op = 1; +namespace +{ + std::atomic<bool> futex_clock_realtime_unavailable; +} + namespace std _GLIBCXX_VISIBILITY(default) { _GLIBCXX_BEGIN_NAMESPACE_VERSION @@ -58,6 +66,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION } else { + if (!futex_clock_realtime_unavailable.load(std::memory_order_relaxed)) + { + struct timespec rt; + rt.tv_sec = __s.count(); + rt.tv_nsec = __ns.count(); + if (syscall (SYS_futex, __addr, futex_wait_bitset_op | futex_clock_realtime_flag, __val, &rt, nullptr, futex_bitset_match_any) == -1) + { + _GLIBCXX_DEBUG_ASSERT(errno == EINTR || errno == EAGAIN + || errno == ETIMEDOUT || errno == ENOSYS);
I've just replaced the use of _GLIBCXX_DEBUG_ASSERT in that file with __glibcxx_assert, because the former is only used when Debug Mode is active, and we'll never build libstdc++.so with Debug Mode. The latter is enabled by -D_GLIBCXX_ASSERTIONS, which I've just added to the default flags for the --enable-libstdcxx-debug option (which builds a second copy of libstdc++.so built without optimisations). I can make the corresponding changes to your patches, so this is just FYI.
