On 13/03/15 13:46 +0000, Jonathan Wakely wrote:
On 12/03/15 18:49 +0000, Jonathan Wakely wrote:
I assumed that Pthreads was enough to ensure pthread_rwlock_t but
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64847 shows that isn't
true for HPUX (seems it was optional prior to POSIX 1003.1-2001).
This adds an autoconf check to decide whether to use pthread_rwlock_t
or the fallback implementation in terms of std::condition_variable and
std::mutex.
Actually the autoconf changes aren't needed, as PR 64847 is already
fixed. Maybe it's better to have an explicit check for
pthread_rwlock_t anyway though ... I'll have a think.
Here's what I've committed, it has Torvald's fixes for retrying on
EAGAIN, lengthy comments explaining why we retry, and also the
autoconf macros (as having a _GLIBCXX_USE_PTHREAD_RWLOCK_T macro that
can be explicitly overriden in a per-target c++config.h is probably
useful).
Tested x86_64-linux, committed to trunk.
commit 9ffe38de54cc4f07c818a6d9119ae9ef6d8bdbda
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Fri Mar 13 13:31:36 2015 +0000
2015-03-18 Jonathan Wakely <jwak...@redhat.com>
Torvald Riegel <trie...@redhat.com>
* acinclude.m4 (GLIBCXX_CHECK_GTHREADS): Check for pthread_rwlock_t.
* config.h.in: Regenerate.
* configure: Regenerate.
* include/std/shared_mutex: Check _GLIBCXX_USE_PTHREAD_RWLOCK_T.
(shared_timed_mutex::_M_rwlock): Use PTHREAD_RWLOCK_INITIALIZER.
(shared_timed_mutex::lock_shared()): Retry on EAGAIN.
(shared_timed_mutex::try_lock_shared_until()): Retry on EAGAIN and
EDEADLK.
diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 0b8c0f0..74f5a65 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -3563,6 +3563,13 @@ AC_DEFUN([GLIBCXX_CHECK_GTHREADS], [
if test x"$ac_has_gthreads" = x"yes"; then
AC_DEFINE(_GLIBCXX_HAS_GTHREADS, 1,
[Define if gthreads library is available.])
+
+ # Also check for pthread_rwlock_t for std::shared_timed_mutex in C++14
+ AC_CHECK_TYPE([pthread_rwlock_t],
+ [AC_DEFINE([_GLIBCXX_USE_PTHREAD_RWLOCK_T], 1,
+ [Define if POSIX read/write locks are available in <gthr.h>.])],
+ [],
+ [#include "gthr.h"])
fi
CXXFLAGS="$ac_save_CXXFLAGS"
diff --git a/libstdc++-v3/include/std/shared_mutex b/libstdc++-v3/include/std/shared_mutex
index 5dcc295..ab1b45b 100644
--- a/libstdc++-v3/include/std/shared_mutex
+++ b/libstdc++-v3/include/std/shared_mutex
@@ -57,10 +57,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
/// shared_timed_mutex
class shared_timed_mutex
{
-#if defined(__GTHREADS_CXX0X)
+#ifdef _GLIBCXX_USE_PTHREAD_RWLOCK_T
typedef chrono::system_clock __clock_t;
- pthread_rwlock_t _M_rwlock;
+#ifdef PTHREAD_RWLOCK_INITIALIZER
+ pthread_rwlock_t _M_rwlock = PTHREAD_RWLOCK_INITIALIZER;
+
+ public:
+ shared_timed_mutex() = default;
+ ~shared_timed_mutex() = default;
+#else
+ pthread_rwlock_t _M_rwlock;
public:
shared_timed_mutex()
@@ -82,6 +89,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// Errors not handled: EBUSY, EINVAL
_GLIBCXX_DEBUG_ASSERT(__ret == 0);
}
+#endif
shared_timed_mutex(const shared_timed_mutex&) = delete;
shared_timed_mutex& operator=(const shared_timed_mutex&) = delete;
@@ -165,12 +173,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
void
lock_shared()
{
- int __ret = pthread_rwlock_rdlock(&_M_rwlock);
+ int __ret;
+ // We retry if we exceeded the maximum number of read locks supported by
+ // the POSIX implementation; this can result in busy-waiting, but this
+ // is okay based on the current specification of forward progress
+ // guarantees by the standard.
+ do
+ __ret = pthread_rwlock_rdlock(&_M_rwlock);
+ while (__ret == EAGAIN);
if (__ret == EDEADLK)
__throw_system_error(int(errc::resource_deadlock_would_occur));
- if (__ret == EAGAIN)
- // Maximum number of read locks has been exceeded.
- __throw_system_error(int(errc::device_or_resource_busy));
// Errors not handled: EINVAL
_GLIBCXX_DEBUG_ASSERT(__ret == 0);
}
@@ -210,11 +222,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static_cast<long>(__ns.count())
};
- int __ret = pthread_rwlock_timedrdlock(&_M_rwlock, &__ts);
- // If the maximum number of read locks has been exceeded, or we would
- // deadlock, we just fail to acquire the lock. Unlike for lock(),
- // we are not allowed to throw an exception.
- if (__ret == ETIMEDOUT || __ret == EAGAIN || __ret == EDEADLK)
+ int __ret;
+ // Unlike for lock(), we are not allowed to throw an exception so if
+ // the maximum number of read locks has been exceeded, or we would
+ // deadlock, we just try to acquire the lock again (and will time out
+ // eventually).
+ // In cases where we would exceed the maximum number of read locks
+ // throughout the whole time until the timeout, we will fail to
+ // acquire the lock even if it would be logically free; however, this
+ // is allowed by the standard, and we made a "strong effort"
+ // (see C++14 30.4.1.4p26).
+ // For cases where the implementation detects a deadlock we
+ // intentionally block and timeout so that an early return isn't
+ // mistaken for a spurious failure, which might help users realise
+ // there is a deadlock.
+ do
+ __ret = pthread_rwlock_timedrdlock(&_M_rwlock, &__ts);
+ while (__ret == EAGAIN || __ret == EDEADLK);
+ if (__ret == ETIMEDOUT)
return false;
// Errors not handled: EINVAL
_GLIBCXX_DEBUG_ASSERT(__ret == 0);
@@ -241,7 +266,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
unlock();
}
-#else // defined(__GTHREADS_CXX0X)
+#else // ! _GLIBCXX_USE_PTHREAD_RWLOCK_T
#if _GTHREAD_USE_MUTEX_TIMEDLOCK
struct _Mutex : mutex, __timed_mutex_impl<_Mutex>
@@ -438,7 +463,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_gate1.notify_one();
}
}
-#endif // !defined(__GTHREADS_CXX0X)
+#endif // ! _GLIBCXX_USE_PTHREAD_RWLOCK_T
};
#endif // _GLIBCXX_HAS_GTHREADS
