linux-kernel - Re: [RFC] LKMM: Add volatile

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Message-ID: <20210604204316.GA4397@paulmck-ThinkPad-P17-Gen-1>
Date:   Fri, 4 Jun 2021 13:43:16 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Segher Boessenkool <segher@...nel.crashing.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nick Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()

On Fri, Jun 04, 2021 at 09:17:56PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 04, 2021 at 12:24:07PM -0500, Segher Boessenkool wrote:
> > On Fri, Jun 04, 2021 at 10:10:29AM -0700, Linus Torvalds wrote:
> > > The compiler *cannot* just say "oh, I'll do that 'volatile asm
> > > barrier' whether the condition is true or not". That would be a
> > > fundamental compiler bug.
> > 
> > Yes.
> 
> So we can all agree on something like this?
> 
> #define volatile_if(x) \
> 	if (({ _Bool __x = (x); BUILD_BUG_ON(__builtin_constant_p(__x)); __x; }) && \
> 	    ({ barrier(); 1; }))

As long as this prevents compilers from causing trouble with things like
conditional-move instructions, I am good.  I don't know that this trouble
actually exists, but I never have been able to get official confirmation
one way or the other.  :-/

> Do we keep volatile_if() or do we like ctrl_dep_if() better?

I like ctrl_dep_if() because that is what it does, but I don't feel all
that strongly about it.

							Thanx, Paul