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

[Jonathan Wakely <jwakely@redhat.com>]

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 <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  François 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>
>--
>2.11.0
>
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