From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 28DD4C4363A for ; Mon, 5 Oct 2020 14:23:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EAA8F207F7 for ; Mon, 5 Oct 2020 14:23:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726429AbgJEOXw (ORCPT ); Mon, 5 Oct 2020 10:23:52 -0400 Received: from netrider.rowland.org ([192.131.102.5]:36845 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1725963AbgJEOXw (ORCPT ); Mon, 5 Oct 2020 10:23:52 -0400 Received: (qmail 377763 invoked by uid 1000); 5 Oct 2020 10:23:51 -0400 Date: Mon, 5 Oct 2020 10:23:51 -0400 From: Alan Stern To: Will Deacon Cc: "Paul E. McKenney" , parri.andrea@gmail.com, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Subject: Re: Litmus test for question from Al Viro Message-ID: <20201005142351.GB376584@rowland.harvard.edu> References: <20201001045116.GA5014@paulmck-ThinkPad-P72> <20201001161529.GA251468@rowland.harvard.edu> <20201001213048.GF29330@paulmck-ThinkPad-P72> <20201003132212.GB318272@rowland.harvard.edu> <20201004233146.GP29330@paulmck-ThinkPad-P72> <20201005023846.GA359428@rowland.harvard.edu> <20201005082002.GA23216@willie-the-truck> <20201005091247.GA23575@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201005091247.GA23575@willie-the-truck> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Mon, Oct 05, 2020 at 10:12:48AM +0100, Will Deacon wrote: > On Mon, Oct 05, 2020 at 09:20:03AM +0100, Will Deacon wrote: > > On Sun, Oct 04, 2020 at 10:38:46PM -0400, Alan Stern wrote: > > > Considering the bug in herd7 pointed out by Akira, we should rewrite P1 as: > > > > > > P1(int *x, int *y) > > > { > > > int r2; > > > > > > r = READ_ONCE(*y); > > > > (r2?) > > > > > WRITE_ONCE(*x, r2); > > > } > > > > > > Other than that, this is fine. > > > > But yes, module the typo, I agree that this rewrite is much better than the > > proposal above. The definition of control dependencies on arm64 (per the Arm > > ARM [1]) isn't entirely clear that it provides order if the WRITE is > > executed on both paths of the branch, and I believe there are ongoing > > efforts to try to tighten that up. I'd rather keep _that_ topic separate > > from the "bug in herd" topic to avoid extra confusion. > > Ah, now I see that you're changing P1 here, not P0. So I'm now nervous > about claiming that this is a bug in herd without input from Jade or Luc, > as it does unfortunately tie into the definition of control dependencies > and it could be a deliberate choice. I think you misunderstood. The bug in herd7 affects the way it handles P1, not P0. With r2 = READ_ONCE(*y); WRITE_ONCE(*x, r2); herd7 generates a data dependency from the read to the write. With WRITE_ONCE(*x, READ_ONCE(*y)); it doesn't generate any dependency, even though the code does exactly the same thing as far as the memory model is concerned. That's the bug I was referring to. The failure to recognize the dependency in P0 should be considered a combined limitation of the memory model and herd7. It's not a simple mistake that can be fixed by a small rewrite of herd7; rather it's a deliberate choice we made based on herd7's inherent design. We explicitly said that control dependencies extend only to the code in the branches of an "if" statement; anything beyond the end of the statement is not considered to be dependent. Alan