From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [PATCH net-next] af_unix: fix a fatal race with bit fields Date: Fri, 03 May 2013 07:14:15 -0700 Message-ID: <1367590455.29805.30.camel@edumazet-glaptop> References: <1367370761.11020.22.camel@edumazet-glaptop> <20130501115103.58e40f37@kryten> <1367375060.11020.24.camel@edumazet-glaptop> <20130501035425.GD5221@bubble.grove.modra.org> <1367384672.11020.34.camel@edumazet-glaptop> <20130503013136.GN5221@bubble.grove.modra.org> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: Anton Blanchard , David Miller , netdev , linuxppc-dev@lists.ozlabs.org, Paul Mackerras , Ambrose Feinstein To: Alan Modra Return-path: Received: from mail-pa0-f52.google.com ([209.85.220.52]:56602 "EHLO mail-pa0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932690Ab3ECOOX (ORCPT ); Fri, 3 May 2013 10:14:23 -0400 Received: by mail-pa0-f52.google.com with SMTP id bg2so945462pad.25 for ; Fri, 03 May 2013 07:14:22 -0700 (PDT) In-Reply-To: <20130503013136.GN5221@bubble.grove.modra.org> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, 2013-05-03 at 11:01 +0930, Alan Modra wrote: > On Tue, Apr 30, 2013 at 10:04:32PM -0700, Eric Dumazet wrote: > > These kind of errors are pretty hard to find, its a pity to spend time > > on them. > > Well, yes. From the first comment in gcc PR52080. "For the following > testcase we generate a 8 byte RMW cycle on IA64 which causes locking > problems in the linux kernel btrfs filesystem." > > Did someone fix btrfs, but not check other kernel locks? Having now > hit the same problem again, have you checked that other kernel locks > don't have adjacent bit fields in the same 64-bit word? And comment > the struct to ensure someone doesn't optimize those unsigned chars > back to bit fields. Not only spinlock, but atomic_t followed by bit fields. BTW, if a spinlock is followed by bit fields, but bit fields only changed when this spinlock is held, there is no problem, unless spinlock is a ticket spinlock. In af_unix, bug happens because the bit fields were changed without spinlock being held (another global spinlock is used instead) (ppc64 doesnt use ticket spinlocks yet)