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

[Torvald Riegel <triegel at redhat dot com>]

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?

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