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Re: [patch] Fix shared_timed_mutex::try_lock_until() et al
- From: Jonathan Wakely <jwakely at redhat dot com>
- To: Torvald Riegel <triegel at redhat dot com>
- Cc: libstdc++ at gcc dot gnu dot org, gcc-patches at gcc dot gnu dot org
- Date: Fri, 10 Apr 2015 09:37:57 +0100
- Subject: Re: [patch] Fix shared_timed_mutex::try_lock_until() et al
- Authentication-results: sourceware.org; auth=none
- References: <20150407142830 dot GG9755 at redhat dot com> <1428512373 dot 924 dot 26 dot camel at triegel dot csb> <20150408191159 dot GQ9755 at redhat dot com> <20150408193808 dot GR9755 at redhat dot com> <1428624966 dot 924 dot 32 dot camel at triegel dot csb>
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
>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
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
index 7f26465..351a4f6 100644
@@ -57,7 +57,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
+#if defined(_GLIBCXX_USE_PTHREAD_RWLOCK_T) &&
typedef chrono::system_clock __clock_t;