[PATCH] Strengthen memory memory order for atomic<T>::wait/notify

[Thomas Rodgers <trodgers@redhat.com>]

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This patch updates the memory order of atomic accesses to the waiter's
count to match libc++'s usage. It should be backported to GCC11.

Tested x86_64-pc-linux-gnu.

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From f5ed7674f86283db4f4ff49a2cc65d4f852413a1 Mon Sep 17 00:00:00 2001
From: Thomas Rodgers <rodgert@twrodgers.com>
Date: Sat, 15 Jan 2022 17:40:49 -0800
Subject: [PATCH] Strengthen memory memory order for atomic<T>::wait/notify

This matches the memory order in libc++.

libstdc++-v3/ChangeLog:
	* libstdc++-v3/include/bits/atomic_wait.h: Change memory order
	from Acquire/Release with relaxed loads to SeqCst+Release for
	accesses to the waiter's count.
---
 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 05cf0013d2a..d7de0d7eb9e 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -209,18 +209,18 @@ _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
@@ -258,7 +258,7 @@ _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);
@@ -309,7 +309,7 @@ _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);
-- 
2.31.1