libstdc++: Strengthen memory order for atomic<T>::wait/notify

Thomas Rodgers · Thomas Rodgers · commit 0c30241b331e · 2022-02-10T10:21:55.000-08:00
This changes the memory order used in the spin wait code to match that of libc++. libstdc++-v3/ChangeLog: * include/bits/atomic_wait.h (__waiter_base::_S_do_spin, __waiter_base::_S_do_spin_v): Change memory order from relaxed to acquire. (cherry picked from commit b25a7c0)
diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
@@ -334,7 +334,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else
 	      {
-		__atomic_load(__addr, &__val, __ATOMIC_RELAXED);
+		__atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
 	      }
 	    return __atomic_spin(__pred, __spin);
 	  }
@@ -355,7 +355,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		     __platform_wait_t& __val,
 		     _Spin __spin = _Spin{ })
 	  {
-	    __atomic_load(__addr, &__val, __ATOMIC_RELAXED);
+	    __atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
 	    return __atomic_spin(__pred, __spin);
 	  }
 

Original file line number	Diff line number	Diff line change
`@@ -334,7 +334,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION`
`334`	`334`	`}`
`335`	`335`	`else`
`336`	`336`	`{`
`337`		`- __atomic_load(__addr, &__val, __ATOMIC_RELAXED);`
	`337`	`+ __atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);`
`338`	`338`	`}`
`339`	`339`	`return __atomic_spin(__pred, __spin);`
`340`	`340`	`}`
`@@ -355,7 +355,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION`
`355`	`355`	`__platform_wait_t& __val,`
`356`	`356`	`_Spin __spin = _Spin{ })`
`357`	`357`	`{`
`358`		`- __atomic_load(__addr, &__val, __ATOMIC_RELAXED);`
	`358`	`+ __atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);`
`359`	`359`	`return __atomic_spin(__pred, __spin);`
`360`	`360`	`}`
`361`	`361`