From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [RFC][PATCH 0/5] arch: atomic rework Date: Fri, 14 Feb 2014 09:29:20 -0800 Message-ID: <20140214172920.GQ4250@linux.vnet.ibm.com> References: <20140207180216.GP4250@linux.vnet.ibm.com> <1391992071.18779.99.camel@triegel.csb> <1392183564.18779.2187.camel@triegel.csb> <20140212180739.GB4250@linux.vnet.ibm.com> <20140213002355.GI4250@linux.vnet.ibm.com> <1392321837.18779.3249.camel@triegel.csb> <20140214020144.GO4250@linux.vnet.ibm.com> <1392352981.18779.3800.camel@triegel.csb> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from e31.co.us.ibm.com ([32.97.110.149]:41215 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752134AbaBNR32 (ORCPT ); Fri, 14 Feb 2014 12:29:28 -0500 Received: from /spool/local by e31.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 14 Feb 2014 10:29:27 -0700 Content-Disposition: inline In-Reply-To: <1392352981.18779.3800.camel@triegel.csb> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Torvald Riegel Cc: Linus Torvalds , Will Deacon , Peter Zijlstra , Ramana Radhakrishnan , David Howells , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "akpm@linux-foundation.org" , "mingo@kernel.org" , "gcc@gcc.gnu.org" On Thu, Feb 13, 2014 at 08:43:01PM -0800, Torvald Riegel wrote: > On Thu, 2014-02-13 at 18:01 -0800, Paul E. McKenney wrote: [ . . . ] > > Another option would be to flag the conditional expression, prohibiting > > the compiler from optimizing out any conditional branches. Perhaps > > something like this: > > > > r1 = atomic_load(x, memory_order_control); > > if (control_dependency(r1)) > > atomic_store(y, memory_order_relaxed); > > That's the one I had in mind and talked to you about earlier today. My > gut feeling is that this is preferably over the other because it "marks" > the if-statement, so the compiler knows exactly which branches matter. > I'm not sure one would need the other memory order for that, if indeed > all you want is relaxed -> branch -> relaxed. But maybe there are > corner cases (see the weaker-than-relaxed discussion in SG1 today). Linus, Peter, any objections to marking places where we are relying on ordering from control dependencies against later stores? This approach seems to me to have significant documentation benefits. Thanx, Paul