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

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Message-ID: <CAHk-=wjf-VJZd3Uxv3T3pSJYYVzyfK2--znG0VEOnNRchMGgdQ@mail.gmail.com>
Date:   Fri, 4 Jun 2021 10:10:29 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.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 4, 2021 at 9:37 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> >
> > Why is "volatile_if()" not just
> >
> >        #define barier_true() ({ barrier(); 1; })
> >
> >        #define volatile_if(x) if ((x) && barrier_true())
>
> Because we weren't sure compilers weren't still allowed to optimize the
> branch away.

This isn't about some "compiler folks think".

The above CANNOT be compiled any other way than with a branch.

A compiler that optimizes a branch away is simply broken.

Of course, the actual condition (ie "x" above) has to be something
that the compiler cannot statically determine is a constant, but since
the whole - and only - point is that there will be a READ_ONCE() or
similar there, that's not an issue.

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.

It's as if we wrote

    if (x) y++;

and the compiler went "Oh, I'll just increment 'y' unconditionally by
one, I'm sure the programmer doesn't mind, the conditional on 'x' is
immaterial".

No. That's not a C compiler. That's a stinking piece of buggy shit.
The compiler has to honor the conditional.

In that "y++" case, a compiler can decide to do it without a branch,
and basically rewrite the above as

   y += !!x;

but with a "volatile asm", that would be a bug.

Of course, we might want to make sure that the compiler doesn't go
"oh, empty asm, I can ignore it", but if that's the case then it's not
about "volatile_if()" any more, at that point it's "oh, the compiler
broke our 'barrier()' implementation", and we have bigger issues.

              Linus