On 10/07/18 11:09 +0100, Mike Crowe wrote:
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.
The patch looks good, thanks.
Can we come up with a test for this, using a user-defined clock that
jumps back?
[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(-)
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 <m...@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 François Dumont <fdum...@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>
--
2.11.0
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