[backport proposed 4.8 fix for __cxa_guard_aqcuire <mikpe@it.uu.se>]

From: Thiago Macieira <thiago dot macieira at intel dot com>
Subject: [PATCH, libstdc++] Use acquire semantics in case of CAS failure
Date: Thu, 30 Aug 2012 12:47:50 +0200
List-Id: <gcc-patches.gcc.gnu.org>

Hello

I detected this issue as I was updating the patches to send to the mailing
list. I have not created a bug report.

When the CAS operation fails and expected == guard_bit, __cxa_guard_acquire
will return immediately indicating that the initialisation has already
succeeded. However, it's missing the acquire barrier for the changes done on
the other thread, to match the release barrier from __cxa_guard_release.

That is:
thread A					thread B
load.acq == 0				load.acq == 0
__cxa_guard_acquire			__cxa_guard_acquire
CAS(0 -> 256) success
__cxa_guard_release
store.rel(1)
							CAS(0 ->256) fails

At this point, we must synchronise with the store-release from thread A.

2012-08-30  Thiago Macieira <thiago.macieira@intel.com>

	* libsupc++/guard.cc (__cxa_guard_acquire): must use acquire semantics
	in case of failure, to acquire changes done by the other thread

--- gcc-4.7.1/libstdc++-v3/libsupc++/guard.cc.~1~	2012-09-08 22:13:06.000000000 +0200
+++ gcc-4.7.1/libstdc++-v3/libsupc++/guard.cc	2012-09-08 22:16:41.000000000 +0200
@@ -253,7 +253,7 @@ namespace __cxxabiv1 
 	    int expected(0);
 	    if (__atomic_compare_exchange_n(gi, &expected, pending_bit, false,
 					    __ATOMIC_ACQ_REL,
-					    __ATOMIC_RELAXED))
+					    __ATOMIC_ACQUIRE))
 	      {
 		// This thread should do the initialization.
 		return 1;