[gcc r11-9529] Strengthen memory order for atomic<T>::wait/notify

[gcc r11-9529] Strengthen memory order for atomic<T>::wait/notify

[Thomas Rodgers]

https://gcc.gnu.org/g:f0343391816c34e85d378131ec436d2145befefb

commit r11-9529-gf0343391816c34e85d378131ec436d2145befefb
Author: Thomas Rodgers <rodgert@appliantology.com>
Date:   Mon Jan 31 13:39:44 2022 -0800

    Strengthen memory order for atomic<T>::wait/notify
    
    This matches the memory order in libc++.
    
    libstdc++-v3/ChangeLog:
            * include/bits/atomic_wait.h: Change memory order from
            Acquire/Release with relaxed loads to SeqCst+Release for
            accesses to the waiter's count.
    
    (cherry picked from commit 07a971b28c880938bb7e070465ab8ee6ccdad1fb)

Diff:
---
 libstdc++-v3/include/bits/atomic_wait.h | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index 35c92644146..f6a1c0d8c32 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -211,18 +211,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       void
       _M_enter_wait() noexcept
-      { __atomic_fetch_add(&_M_wait, 1, __ATOMIC_ACQ_REL); }
+      { __atomic_fetch_add(&_M_wait, 1, __ATOMIC_SEQ_CST); }
 
       void
       _M_leave_wait() noexcept
-      { __atomic_fetch_sub(&_M_wait, 1, __ATOMIC_ACQ_REL); }
+      { __atomic_fetch_sub(&_M_wait, 1, __ATOMIC_RELEASE); }
 
       bool
       _M_waiting() const noexcept
       {
 	__platform_wait_t __res;
-	__atomic_load(&_M_wait, &__res, __ATOMIC_ACQUIRE);
-	return __res > 0;
+	__atomic_load(&_M_wait, &__res, __ATOMIC_SEQ_CST);
+	return __res != 0;
       }
 
       void
@@ -260,7 +260,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	__platform_wait(__addr, __old);
 #else
 	__platform_wait_t __val;
-	__atomic_load(__addr, &__val, __ATOMIC_RELAXED);
+	__atomic_load(__addr, &__val, __ATOMIC_SEQ_CST);
 	if (__val == __old)
 	  {
 	    lock_guard<mutex> __l(_M_mtx);
@@ -311,7 +311,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	{
 	  if (_M_laundered())
 	    {
-	      __atomic_fetch_add(_M_addr, 1, __ATOMIC_ACQ_REL);
+	      __atomic_fetch_add(_M_addr, 1, __ATOMIC_SEQ_CST);
 	      __all = true;
 	    }
 	  _M_w._M_notify(_M_addr, __all, __bare);