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

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

[Mike Crowe]

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(-)

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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[PATCH 2/2] condition_variable: Use steady_clock to implement wait_for

[Mike Crowe]

I believe[1][2] that the C++ standard says that
std::condition_variable::wait_for should be implemented to be equivalent
to:

 return wait_until(lock, chrono::steady_clock::now() + rel_time);

But the existing implementation uses chrono::system_clock. Now that
wait_until has potentially-different behaviour for chrono::steady_clock,
let's at least try to wait using the correct clock.

[1] https://en.cppreference.com/w/cpp/thread/condition_variable/wait_for
[2] https://github.com/cplusplus/draft/blob/master/source/threads.tex
---
 libstdc++-v3/ChangeLog                      | 3 +++
 libstdc++-v3/include/std/condition_variable | 5 +++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index ea7875ace9f..4500273ace7 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,4 +1,7 @@
 2018-07-09  Mike Crowe <mac@mcrowe.com>
+       * include/std/condition_variable (wait_for): Use steady_clock.
+
+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.
diff --git a/libstdc++-v3/include/std/condition_variable b/libstdc++-v3/include/std/condition_variable
index a2d146a9b09..ce583990b9d 100644
--- a/libstdc++-v3/include/std/condition_variable
+++ b/libstdc++-v3/include/std/condition_variable
@@ -65,6 +65,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   class condition_variable
   {
     typedef chrono::system_clock       __clock_t;
+    typedef chrono::steady_clock       __steady_clock_t;
     typedef __gthread_cond_t           __native_type;

 #ifdef __GTHREAD_COND_INIT
@@ -142,11 +143,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       wait_for(unique_lock<mutex>& __lock,
               const chrono::duration<_Rep, _Period>& __rtime)
       {
-       using __dur = typename __clock_t::duration;
+       using __dur = typename __steady_clock_t::duration;
        auto __reltime = chrono::duration_cast<__dur>(__rtime);
        if (__reltime < __rtime)
          ++__reltime;
-       return wait_until(__lock, __clock_t::now() + __reltime);
+       return wait_until(__lock, __steady_clock_t::now() + __reltime);
       }

     template<typename _Rep, typename _Period, typename _Predicate>
--
2.11.0

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

[Jonathan Wakely]

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

[Mike Crowe]

On Friday 20 July 2018 at 11:53:38 +0100, Jonathan Wakely wrote:
> 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?

That's a good idea. I'll do that.

Thanks.

Mike.

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

[Jonathan Wakely]

On 20/07/18 12:30 +0100, Mike Crowe wrote:
>On Friday 20 July 2018 at 11:53:38 +0100, Jonathan Wakely wrote:
>> 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?
>
>That's a good idea. I'll do that.

Thanks.

There are "broken" clocks in testsuite/30_threads/this_thread/60421.cc
and testsuite/30_threads/timed_mutex/try_lock_until/57641.cc but I'm
not sure if either of them is reusable. A custom one for this test
should be reasonably easy.

Re: [PATCH 2/2] condition_variable: Use steady_clock to implement wait_for

[Jonathan Wakely]

On 20/07/18 17:49 +0100, Mike Crowe wrote:
>I believe[1][2] that the C++ standard says that
>std::condition_variable::wait_for should be implemented to be equivalent
>to:
>
> return wait_until(lock, chrono::steady_clock::now() + rel_time);
>
>But the existing implementation uses chrono::system_clock. Now that
>wait_until has potentially-different behaviour for chrono::steady_clock,
>let's at least try to wait using the correct clock.
>
>[1] https://en.cppreference.com/w/cpp/thread/condition_variable/wait_for
>[2] https://github.com/cplusplus/draft/blob/master/source/threads.tex

Also committed to trunk. Thanks again.

>---
> libstdc++-v3/ChangeLog                      | 3 +++
> libstdc++-v3/include/std/condition_variable | 5 +++--
> 2 files changed, 6 insertions(+), 2 deletions(-)
>
>diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
>index 4657af7..432cb84 100644
>--- a/libstdc++-v3/ChangeLog
>+++ b/libstdc++-v3/ChangeLog
>@@ -1,4 +1,7 @@
> 2018-07-20  Mike Crowe <mac@mcrowe.com>
>+       * include/std/condition_variable (wait_for): Use steady_clock.
>+
>+2018-07-20  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.
>diff --git a/libstdc++-v3/include/std/condition_variable b/libstdc++-v3/include/std/condition_variable
>index a2d146a..ce58399 100644
>--- a/libstdc++-v3/include/std/condition_variable
>+++ b/libstdc++-v3/include/std/condition_variable
>@@ -65,6 +65,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>   class condition_variable
>   {
>     typedef chrono::system_clock       __clock_t;
>+    typedef chrono::steady_clock       __steady_clock_t;
>     typedef __gthread_cond_t           __native_type;
>
> #ifdef __GTHREAD_COND_INIT
>@@ -142,11 +143,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       wait_for(unique_lock<mutex>& __lock,
>               const chrono::duration<_Rep, _Period>& __rtime)
>       {
>-       using __dur = typename __clock_t::duration;
>+       using __dur = typename __steady_clock_t::duration;
>        auto __reltime = chrono::duration_cast<__dur>(__rtime);
>        if (__reltime < __rtime)
>          ++__reltime;
>-       return wait_until(__lock, __clock_t::now() + __reltime);
>+       return wait_until(__lock, __steady_clock_t::now() + __reltime);
>       }
>
>     template<typename _Rep, typename _Period, typename _Predicate>
>--
>git-series 0.9.1
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[PATCH 2/2] condition_variable: Use steady_clock to implement wait_for

[Mike Crowe]

I believe[1][2] that the C++ standard says that
std::condition_variable::wait_for should be implemented to be equivalent
to:

 return wait_until(lock, chrono::steady_clock::now() + rel_time);

But the existing implementation uses chrono::system_clock. Now that
wait_until has potentially-different behaviour for chrono::steady_clock,
let's at least try to wait using the correct clock.

[1] https://en.cppreference.com/w/cpp/thread/condition_variable/wait_for
[2] https://github.com/cplusplus/draft/blob/master/source/threads.tex
---
 libstdc++-v3/ChangeLog                      | 3 +++
 libstdc++-v3/include/std/condition_variable | 5 +++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 4657af7..432cb84 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,4 +1,7 @@
 2018-07-20  Mike Crowe <mac@mcrowe.com>
+       * include/std/condition_variable (wait_for): Use steady_clock.
+
+2018-07-20  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.
diff --git a/libstdc++-v3/include/std/condition_variable b/libstdc++-v3/include/std/condition_variable
index a2d146a..ce58399 100644
--- a/libstdc++-v3/include/std/condition_variable
+++ b/libstdc++-v3/include/std/condition_variable
@@ -65,6 +65,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   class condition_variable
   {
     typedef chrono::system_clock       __clock_t;
+    typedef chrono::steady_clock       __steady_clock_t;
     typedef __gthread_cond_t           __native_type;

 #ifdef __GTHREAD_COND_INIT
@@ -142,11 +143,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       wait_for(unique_lock<mutex>& __lock,
               const chrono::duration<_Rep, _Period>& __rtime)
       {
-       using __dur = typename __clock_t::duration;
+       using __dur = typename __steady_clock_t::duration;
        auto __reltime = chrono::duration_cast<__dur>(__rtime);
        if (__reltime < __rtime)
          ++__reltime;
-       return wait_until(__lock, __clock_t::now() + __reltime);
+       return wait_until(__lock, __steady_clock_t::now() + __reltime);
       }

     template<typename _Rep, typename _Period, typename _Predicate>
--
git-series 0.9.1
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