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

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Message-ID: <20210604191756.GE68208@worktop.programming.kicks-ass.net>
Date:   Fri, 4 Jun 2021 21:17:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...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 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; }))

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