From ae2f6e01749f39b8b8ccc211fc29e5e6ec8b57cd Mon Sep 17 00:00:00 2001
From: Thomas Rodgers <rodgert@twrodgers.com>
Date: Wed, 21 Apr 2021 18:12:03 -0700
Subject: [PATCH] libstdc++: Fix "bare" notifications dropped by waiters check

For types that track whether or not there extant waiters (e.g.
semaphore) internally, the __atomic_notify_address_bare() call was
introduced to avoid the overhead of loading the atomic count of
waiters. For platforms that don't have Futex, however, there was
still a check for waiters, and seeing that there are none (because
in the bare case, the count is not incremented), the notification
is dropped. This commit addresses that case.

libstdc++-v3/ChangeLog:
	* include/bits/atomic_wait.h: Always notify waiters in the
	case of 'bare' address notification.
---
 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 0ac5575190c6..984ed70f16c9 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -226,9 +226,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
       void
-      _M_notify(const __platform_wait_t* __addr, bool __all) noexcept
+      _M_notify(const __platform_wait_t* __addr, bool __all, bool __bare) noexcept
       {
-	if (!_M_waiting())
+	if (!(__bare || _M_waiting()))
 	  return;
 
 #ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
@@ -304,11 +304,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  }
 
 	void
-	_M_notify(bool __all)
+	_M_notify(bool __all, bool __bare = false)
 	{
 	  if (_M_addr == &_M_w._M_ver)
 	    __atomic_fetch_add(_M_addr, 1, __ATOMIC_ACQ_REL);
-	  _M_w._M_notify(_M_addr, __all);
+	  _M_w._M_notify(_M_addr, __all, __bare);
 	}
 
 	template<typename _Up, typename _ValFn,
@@ -452,7 +452,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     __atomic_notify_address(const _Tp* __addr, bool __all) noexcept
     {
       __detail::__bare_wait __w(__addr);
-      __w._M_notify(__all);
+      __w._M_notify(__all, true);
     }
 
   // This call is to be used by atomic types which track contention externally
@@ -464,7 +464,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     __detail::__platform_notify(__addr, __all);
 #else
     __detail::__bare_wait __w(__addr);
-    __w._M_notify(__all);
+    __w._M_notify(__all, true);
 #endif
   }
 _GLIBCXX_END_NAMESPACE_VERSION
-- 
2.47.2