From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 24906 invoked by alias); 5 Aug 2012 19:36:42 -0000 Received: (qmail 24893 invoked by uid 22791); 5 Aug 2012 19:36:41 -0000 X-SWARE-Spam-Status: No, hits=-4.3 required=5.0 tests=ALL_TRUSTED,AWL,BAYES_00,KHOP_THREADED X-Spam-Check-By: sourceware.org Received: from localhost (HELO gcc.gnu.org) (127.0.0.1) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Sun, 05 Aug 2012 19:36:25 +0000 From: "redi at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug libstdc++/54172] [4.7/4.8 Regression] __cxa_guard_acquire thread-safety issue Date: Sun, 05 Aug 2012 19:36:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: libstdc++ X-Bugzilla-Keywords: X-Bugzilla-Severity: major X-Bugzilla-Who: redi at gcc dot gnu.org X-Bugzilla-Status: NEW X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: 4.7.2 X-Bugzilla-Changed-Fields: Status Last reconfirmed Ever Confirmed Message-ID: In-Reply-To: References: X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated Content-Type: text/plain; charset="UTF-8" MIME-Version: 1.0 Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-bugs-owner@gcc.gnu.org X-SW-Source: 2012-08/txt/msg00273.txt.bz2 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54172 Jonathan Wakely changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW Last reconfirmed| |2012-08-05 Ever Confirmed|0 |1 --- Comment #1 from Jonathan Wakely 2012-08-05 19:36:24 UTC --- The analysis looks correct, and I think the patch is actually an improvement on the old behaviour, correctly setting 'expected' to 0 for the first CAS but also avoiding unnecessarily looping when the second CAS fails: we don't need to loop and do the first CAS again to decide what to do, examining the result of that second CAS tells you what needs to be done. I don't particularly like the variable name 'expected' because it serves two purposes: it holds the expected value and after the CAS it holds the actual value, the name 'expected' only suits the first usage and makes comparisons such as (expected == guard_bit) look odd. It's not the expected value that's being compared, it's the actual value which was *not* what was expected. I preferred the original name "old", or maybe better yet "current".