[v5,6/8] libstdc++ atomic_futex: Avoid rounding errors in std::future::wait_* [PR91486]

Message ID fc1cce85a6a8b4d39ff6826bea999edd2381911b.1590732962.git-series.mac@mcrowe.com
State New
Headers show
Series
  • std::future::wait_* and std::condition_variable improvements
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Commit Message

Kees Cook via Gcc-patches May 29, 2020, 6:17 a.m.
Convert the specified duration to the target clock's duration type
before adding it to the current time in
__atomic_futex_unsigned::_M_load_when_equal_for and
_M_load_when_equal_until.  This removes the risk of the timeout being
rounded down to the current time resulting in there being no wait at
all when the duration type lacks sufficient precision to hold the
steady_clock current time.

Rather than using the style of fix from PR68519, let's expose the C++17
std::chrono::ceil function as std::chrono::__detail::ceil so that it can
be used in code compiled with earlier standards versions and simplify
the fix. This was suggested by John Salmon in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91486#c5 .

This problem has become considerably less likely to trigger since I
switched the __atomic__futex_unsigned::__clock_t reference clock from
system_clock to steady_clock and added the loop, but the consequences of
triggering it have changed too.

By my calculations it takes just over 194 days from the epoch for the
current time not to be representable in a float. This means that
system_clock is always subject to the problem (with the standard 1970
epoch) whereas steady_clock with float duration only runs out of
resolution machine has been running for that long (assuming the Linux
implementation of CLOCK_MONOTONIC.)

The recently-added loop in
__atomic_futex_unsigned::_M_load_when_equal_until turns this scenario
into a busy wait.

Unfortunately the combination of both of these things means that it's
not possible to write a test case for this occurring in
_M_load_when_equal_until as it stands.

	* libstdc++-v3/include/std/chrono: (__detail::ceil) Move
          implementation of std::chrono::ceil into private namespace so
          that it's available to pre-C++17 code.

	* libstdc++-v3/include/bits/atomic_futex.h:
	  (__atomic_futex_unsigned::_M_load_when_equal_for,
	  __atomic_futex_unsigned::_M_load_when_equal_until): Use
	  __detail::ceil to convert delta to the reference clock
	  duration type to avoid resolution problems

	* libstdc++-v3/testsuite/30_threads/async/async.cc: (test_pr91486):
          New test for __atomic_futex_unsigned::_M_load_when_equal_for.
---
 libstdc++-v3/include/bits/atomic_futex.h         |  6 +++--
 libstdc++-v3/include/std/chrono                  | 19 +++++++++++++----
 libstdc++-v3/testsuite/30_threads/async/async.cc | 15 +++++++++++++-
 3 files changed, 34 insertions(+), 6 deletions(-)

-- 
git-series 0.9.1

Patch

diff --git a/libstdc++-v3/include/bits/atomic_futex.h b/libstdc++-v3/include/bits/atomic_futex.h
index 5f95ade..aa137a7 100644
--- a/libstdc++-v3/include/bits/atomic_futex.h
+++ b/libstdc++-v3/include/bits/atomic_futex.h
@@ -219,8 +219,9 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _M_load_when_equal_for(unsigned __val, memory_order __mo,
 	  const chrono::duration<_Rep, _Period>& __rtime)
       {
+	using __dur = typename __clock_t::duration;
 	return _M_load_when_equal_until(__val, __mo,
-					__clock_t::now() + __rtime);
+		    __clock_t::now() + chrono::__detail::ceil<__dur>(__rtime));
       }
 
     // Returns false iff a timeout occurred.
@@ -233,7 +234,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	do {
 	  const __clock_t::time_point __s_entry = __clock_t::now();
 	  const auto __delta = __atime - __c_entry;
-	  const auto __s_atime = __s_entry + __delta;
+	  const auto __s_atime = __s_entry +
+	      chrono::__detail::ceil<_Duration>(__delta);
 	  if (_M_load_when_equal_until(__val, __mo, __s_atime))
 	    return true;
 	  __c_entry = _Clock::now();
diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 6d78f32..4257c7c 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -299,6 +299,20 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif
 #endif // C++20
 
+    // We want to use ceil even when compiling for earlier standards versions
+    namespace __detail
+    {
+      template<typename _ToDur, typename _Rep, typename _Period>
+        constexpr __enable_if_is_duration<_ToDur>
+        ceil(const duration<_Rep, _Period>& __d)
+        {
+	  auto __to = chrono::duration_cast<_ToDur>(__d);
+	  if (__to < __d)
+	    return __to + _ToDur{1};
+	  return __to;
+	}
+    }
+
 #if __cplusplus >= 201703L
 # define __cpp_lib_chrono 201611
 
@@ -316,10 +330,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr __enable_if_is_duration<_ToDur>
       ceil(const duration<_Rep, _Period>& __d)
       {
-	auto __to = chrono::duration_cast<_ToDur>(__d);
-	if (__to < __d)
-	  return __to + _ToDur{1};
-	return __to;
+	return __detail::ceil<_ToDur>(__d);
       }
 
     template <typename _ToDur, typename _Rep, typename _Period>
diff --git a/libstdc++-v3/testsuite/30_threads/async/async.cc b/libstdc++-v3/testsuite/30_threads/async/async.cc
index ee117f4..f697292 100644
--- a/libstdc++-v3/testsuite/30_threads/async/async.cc
+++ b/libstdc++-v3/testsuite/30_threads/async/async.cc
@@ -158,6 +158,20 @@  void test04()
   }
 }
 
+void test_pr91486()
+{
+  future<void> f1 = async(launch::async, []() {
+      std::this_thread::sleep_for(std::chrono::seconds(1));
+    });
+
+  std::chrono::duration<float> const wait_time = std::chrono::seconds(1);
+  auto const start_steady = chrono::steady_clock::now();
+  auto status = f1.wait_for(wait_time);
+  auto const elapsed_steady = chrono::steady_clock::now() - start_steady;
+
+  VERIFY( elapsed_steady >= std::chrono::seconds(1) );
+}
+
 int main()
 {
   test01();
@@ -166,5 +180,6 @@  int main()
   test03<std::chrono::steady_clock>();
   test03<steady_clock_copy>();
   test04();
+  test_pr91486();
   return 0;
 }