Re: [PATCH] Strengthen memory memory order for atomic::wait/notify

[Thomas Rodgers via Gcc-patches]

Tested x86_64-pc-linux-gnu, committed to master.
Backported to GCC-11.

On Sun, Jan 16, 2022 at 12:31 PM Jonathan Wakely  wrote:

>
>
> On Sun, 16 Jan 2022 at 01:48, Thomas Rodgers via Libstdc++ <
> libstd...@gcc.gnu.org> wrote:
>
>> 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.
>>
>
> The commit subject line says "memory memory order", OK for trunk and
> gcc-11 with that fixed.
>
>
>> Tested x86_64-pc-linux-gnu.
>>
>

Re: [PATCH] Strengthen memory memory order for atomic::wait/notify

[Jonathan Wakely via Gcc-patches]

On Sun, 16 Jan 2022 at 01:48, Thomas Rodgers via Libstdc++ <
libstd...@gcc.gnu.org> wrote:

> 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.
>

The commit subject line says "memory memory order", OK for trunk and gcc-11
with that fixed.


> Tested x86_64-pc-linux-gnu.
>

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

[Thomas Rodgers via Gcc-patches]

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.
From f5ed7674f86283db4f4ff49a2cc65d4f852413a1 Mon Sep 17 00:00:00 2001
From: Thomas Rodgers 
Date: Sat, 15 Jan 2022 17:40:49 -0800
Subject: [PATCH] Strengthen memory memory order for atomic::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 __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