Re: [patch] Fix shared_timed_mutex::try_lock_until() et al

[Jonathan Wakely <jwakely at redhat dot com>]

On 10/04/15 02:16 +0200, Torvald Riegel wrote:
> On Wed, 2015-04-08 at 20:38 +0100, Jonathan Wakely wrote:
> > On 08/04/15 20:11 +0100, Jonathan Wakely wrote:
> > >We can get rid of the _Mutex type then, and just use std::mutex,
> > >and that also means we can provide the timed locking functions
> > >even when !defined(_GTHREAD_USE_MUTEX_TIMEDLOCK).
> > >
> > >And so maybe we should use this fallback implementation instead of
> > the
> > >pthread_rwlock_t one when !defined(_GTHREAD_USE_MUTEX_TIMEDLOCK),
> > >so that they have a complete std::shared_timed_mutex (this applies
> > >to at least Darwin, not sure which other targets).
> >
> > Here's a further patch to do that (which really needs to go into
> > 5.0 too, so we don't switch Darwin to the new pthread_rwlock_t
> > version and then have to swtich it back again in 6.0).
>
> I understand why a mutex with timeouts isn't required anymore, but
> why do you now add it to the USE_PTHREAD_RWLOCK_T condition?  If
> pthread_rwlock_t is available, why would we need a normal mutex with
> timeouts?

Darwin and HPUX support pthread_rwlock_t but not the timed lock
functions, see https://gcc.gnu.org/PR64847 and (part of)
https://gcc.gnu.org/PR64368 which were "fixed" by
https://gcc.gnu.org/r220161 which just disables the timed lock
functions on those targets, using #if _GTHREAD_USE_MUTEX_TIMEDLOCK.

That means Darwin and HPUX have an incomplete shared_timed_mutex that
doesn't support the timed functions (i.e. what might get added to
C++17 as std::shared_mutex).

The patch below gives Darwin and HPUX a fully conforming (albeit
slower) shared_timed_mutex, by ensuring we don't use pthread_rwlock_t
for targets that can't use timed functions with pthread_rwlock_t.

If std::shared_mutex is added to the standard we could use
pthread_rwlock_t for that even on Darwin and HPUX, because it provides
everything needed for a non-timed std::shared_mutex.

> For example, this chunk here (and others too):
>
> > diff --git a/libstdc++-v3/include/std/shared_mutex b/libstdc
> > ++-v3/include/std/shared_mutex
> > index 7f26465..351a4f6 100644
> > --- a/libstdc++-v3/include/std/shared_mutex
> > +++ b/libstdc++-v3/include/std/shared_mutex
> > @@ -57,7 +57,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >    /// shared_timed_mutex
> >    class shared_timed_mutex
> >    {
> > -#ifdef _GLIBCXX_USE_PTHREAD_RWLOCK_T
> > +#if defined(_GLIBCXX_USE_PTHREAD_RWLOCK_T) &&
> > _GTHREAD_USE_MUTEX_TIMEDLOCK
> >      typedef chrono::system_clock       __clock_t;
> >
> >  #ifdef PTHREAD_RWLOCK_INITIALIZER