libstdc++: Unblock atomic wait on non-futex platforms [PR106183]

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 <trodgers@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.

(cherry picked from commit af98cb88eb4be6a1668ddf966e975149bf8610b1)

diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index a6d55d3af8ae89b9607887bab021ae27cb03a2e2..76ed7409937504d7a21269348f81c5d088149af0 100644 (file)
--- 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>