[gcc r13-1957] libstdc++: Unblock atomic wait on non-futex platforms [PR106183]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:af98cb88eb4be6a1668ddf966e975149bf8610b1

commit r13-1957-gaf98cb88eb4be6a1668ddf966e975149bf8610b1
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jul 28 16:15:58 2022 +0100

    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.

Diff:
---
 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>