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

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Message-ID: <YLpJ5K6O52o1cAVT@hirez.programming.kicks-ass.net>
Date:   Fri, 4 Jun 2021 17:42:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>, paulmck@...nel.org,
        stern@...land.harvard.edu, parri.andrea@...il.com,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com,
        linux-kernel@...r.kernel.org, linux-toolchains@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [RFC] LKMM: Add volatile_if()

On Fri, Jun 04, 2021 at 05:22:04PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 04, 2021 at 04:13:57PM +0100, Will Deacon wrote:
> 
> > In fact, maybe it's actually necessary to bundle the load and branch
> > together. I looked at some of the examples of compilers breaking control
> > dependencies from memory-barriers.txt and the "boolean short-circuit"
> > example seems to defeat volatile_if:
> > 
> > void foo(int *x, int *y)
> > {
> >         volatile_if (READ_ONCE(*x) || 1 > 0)
> >                 WRITE_ONCE(*y, 42);
> > }  
> 
> Yeah, I'm not too bothered about this. Broken is broken.
> 
> If this were a compiler feature, the above would be a compile error. But
> alas, we're not there yet :/ and the best we get to say at this point
> is: don't do that then.

Ha! Fixed it for you:

#define volatile_if(cond) if (({ bool __t = (cond); BUILD_BUG_ON(__builtin_constant_p(__t)); volatile_cond(__t); }))