Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 26 Feb 2019 07:39:57 -0800
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Borislav Petkov <bp@alien8.de>,
        Andrea Parri <andrea.parri@amarulasolutions.com>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Alan Stern <stern@rowland.harvard.edu>,
        Will Deacon <will.deacon@arm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>
Subject: Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo
Reply-To: paulmck@linux.ibm.com
References: <20190225175517.GK4072@linux.ibm.com>
 <20190226093009.GS32477@hirez.programming.kicks-ass.net>
 <20190226104551.GF32534@hirez.programming.kicks-ass.net>
 <20190226112133.GG32534@hirez.programming.kicks-ass.net>
 <20190226112521.GH32534@hirez.programming.kicks-ass.net>
 <20190226113008.GI32534@hirez.programming.kicks-ass.net>
 <20190226113813.GA14753@zn.tnic>
 <20190226134906.GG32494@hirez.programming.kicks-ass.net>
 <20190226142845.GK4072@linux.ibm.com>
 <20190226144730.GV32477@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190226144730.GV32477@hirez.programming.kicks-ass.net>
User-Agent: Mutt/1.5.21 (2010-09-15)
Message-Id: <20190226153957.GN4072@linux.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Feb 26, 2019 at 03:47:30PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 26, 2019 at 06:28:45AM -0800, Paul E. McKenney wrote:
> > I must confess to not being all that sympathetic to code that takes
> > advantage of happenstance stack-frame layout.  Is there some reason
> > we need that?
> 
> Not that I'm aware; but if it gets this 'obvious' case wrong, I worry
> what else it gets wrong.
> 
> At the very least we should get this fixed and compile a kernel with the
> fixed compiler to see what (if anything) changes in the generated code
> and analyse the changes (if any) to make sure we were ok (or not).
> 
> I mean; yes that example is UB, but the result is also clearly batshit
> insane.

My understanding is that their goal is better analysis of pointer
aliasing, and that this case is a side effect.

							Thanx, Paul