Tested x86_64-linux, powerpc64le-linux and sparc-solaris2.11, pushed to trunk.
We want this on the gcc-12 and gcc-11 branches too.
-- >8 --
When using a mutex and condition variable, the notifying thread needs to
increment _M_ver while holding the mutex lock, and the waiting thread
needs to re-check after locking the mutex. This avoids a missed
notification as described in the PR.
By moving the increment of _M_ver to the base _M_notify we can make the
use of the mutex local to the use of the condition variable, and
simplify the code a little. We can use a relaxed store because the mutex
already provides sequential consistency. Also we don't need to check
whether __addr == &_M_ver because we know that's always true for
platforms that use a condition variable, and so we also know that we
always need to use notify_all() not notify_one().
Reviewed-by: Thomas Rodgers <trodg...@redhat.com>
libstdc++-v3/ChangeLog:
PR libstdc++/106183
* include/bits/atomic_wait.h (__waiter_pool_base::_M_notify):
Move increment of _M_ver here.
[!_GLIBCXX_HAVE_PLATFORM_WAIT]: Lock mutex around increment.
Use relaxed memory order and always notify all waiters.
(__waiter_base::_M_do_wait) [!_GLIBCXX_HAVE_PLATFORM_WAIT]:
Check value again after locking mutex.
(__waiter_base::_M_notify): Remove increment of _M_ver.
---
libstdc++-v3/include/bits/atomic_wait.h | 42 ++++++++++++-------------
1 file changed, 20 insertions(+), 22 deletions(-)
diff --git a/libstdc++-v3/include/bits/atomic_wait.h
b/libstdc++-v3/include/bits/atomic_wait.h
index a6d55d3af8a..76ed7409937 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -221,18 +221,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
void
- _M_notify(const __platform_wait_t* __addr, bool __all, bool __bare)
noexcept
+ _M_notify(__platform_wait_t* __addr, [[maybe_unused]] bool __all,
+ bool __bare) noexcept
{
- if (!(__bare || _M_waiting()))
- return;
-
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
- __platform_notify(__addr, __all);
+ if (__addr == &_M_ver)
+ {
+ __atomic_fetch_add(__addr, 1, __ATOMIC_SEQ_CST);
+ __all = true;
+ }
+
+ if (__bare || _M_waiting())
+ __platform_notify(__addr, __all);
#else
- if (__all)
+ {
+ lock_guard<mutex> __l(_M_mtx);
+ __atomic_fetch_add(__addr, 1, __ATOMIC_RELAXED);
+ }
+ if (__bare || _M_waiting())
_M_cv.notify_all();
- else
- _M_cv.notify_one();
#endif
}
@@ -259,7 +266,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
if (__val == __old)
{
lock_guard<mutex> __l(_M_mtx);
- _M_cv.wait(_M_mtx);
+ __atomic_load(__addr, &__val, __ATOMIC_RELAXED);
+ if (__val == __old)
+ _M_cv.wait(_M_mtx);
}
#endif // __GLIBCXX_HAVE_PLATFORM_WAIT
}
@@ -297,20 +306,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
, _M_addr(_S_wait_addr(__addr, &_M_w._M_ver))
{ }
- bool
- _M_laundered() const
- { return _M_addr == &_M_w._M_ver; }
-
void
- _M_notify(bool __all, bool __bare = false)
- {
- if (_M_laundered())
- {
- __atomic_fetch_add(_M_addr, 1, __ATOMIC_SEQ_CST);
- __all = true;
- }
- _M_w._M_notify(_M_addr, __all, __bare);
- }
+ _M_notify(bool __all, bool __bare = false) noexcept
+ { _M_w._M_notify(_M_addr, __all, __bare); }
template<typename _Up, typename _ValFn,
typename _Spin = __default_spin_policy>
--
2.37.1
