[1/2] condition_variable: Report early wakeup of wait_until as no_timeout

Message ID 20180710100929.32704-1-mac@mcrowe.com
State New
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  • [1/2] condition_variable: Report early wakeup of wait_until as no_timeout
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Commit Message

Mike Crowe July 10, 2018, 10:09 a.m.
As currently implemented, condition_variable always ultimately waits
against std::chrono::system_clock. This clock can be changed in arbitrary
ways by the user which may result in us waking up too early or too late
when measured against the caller-supplied clock.

We can't (yet) do much about waking up too late[1], but
if we wake up too early we must return cv_status::no_timeout to indicate a
spurious wakeup rather than incorrectly returning cv_status::timeout.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=41861
---
 libstdc++-v3/ChangeLog                      | 5 +++++
 libstdc++-v3/include/std/condition_variable | 8 +++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

--
2.11.0

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Patch

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index cceef0271ae..ea7875ace9f 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,8 @@ 
+2018-07-09  Mike Crowe <mac@mcrowe.com>
+       * include/std/condition_variable (wait_until): Only report timeout
+       if we really have timed out when measured against the
+       caller-supplied clock.
+
 2018-07-06  Fran├žois Dumont  <fdumont@gcc.gnu.org>

        * include/debug/functions.h (__gnu_debug::__check_string): Move...
diff --git a/libstdc++-v3/include/std/condition_variable b/libstdc++-v3/include/std/condition_variable
index 84863a162d6..a2d146a9b09 100644
--- a/libstdc++-v3/include/std/condition_variable
+++ b/libstdc++-v3/include/std/condition_variable
@@ -116,7 +116,13 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
        const auto __delta = __atime - __c_entry;
        const auto __s_atime = __s_entry + __delta;

-       return __wait_until_impl(__lock, __s_atime);
+       // We might get a timeout when measured against __clock_t but
+       // we need to check against the caller-supplied clock to tell
+       // whether we should return a timeout.
+       if (__wait_until_impl(__lock, __s_atime) == cv_status::timeout)
+         return _Clock::now() < __atime ? cv_status::no_timeout : cv_status::timeout;
+       else
+         return cv_status::no_timeout;
       }

     template<typename _Clock, typename _Duration, typename _Predicate>