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

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Date:   Fri, 4 Jun 2021 18:37:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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 09:30:01AM -0700, Linus Torvalds wrote:
> On Fri, Jun 4, 2021 at 3:12 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > I've converted most architectures we care about, and the rest will get
> > an extra smp_mb() by means of the 'generic' fallback implementation (for
> > now).
> 
> Why is "volatile_if()" not just
> 
>        #define barier_true() ({ barrier(); 1; })
> 
>        #define volatile_if(x) if ((x) && barrier_true())
> 
> because that should essentially cause the same thing - the compiler
> should be *forced* to create one conditional branch (because "barrier"
> is an asm that can't be done on the false side, so it can't do it with
> arithmetic or other games), and after that we're done.
> 
> No need for per-architecture "asm goto" games. No new memory barriers.
> No actual new code generation (except for the empty asm volatile that
> is a barrier).

Because we weren't sure compilers weren't still allowed to optimize the
branch away. If compiler folks can guarantee us your thing (along with
maybe the BUILD_BUG_ON(__builtin_constant_p(cond)) thing) always shall
generate a conditional branch instruction, then Yay!