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

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Date:   Fri, 4 Jun 2021 14:53:01 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        will@...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 11:55:26AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 04, 2021 at 11:40:47AM -0500, Segher Boessenkool wrote:
> > My point is that you ask compiler developers to paint themselves into a
> > corner if you ask them to change such fundamental C syntax.
> 
> Once we have some experience with a language extension, the official
> syntax for a standardized version of that extension can be bikeshedded.
> Committees being what they are, what we use in the meantime will
> definitely not be what is chosen, so there is not a whole lot of point
> in worrying about the exact syntax in the meantime.  ;-)

I am only saying that it is unlikely any compiler that is used in
production will want to experiment with "volatile if".

> > I would love to see something that meshes well with the rest of C.  But
> > there is no 1-1 translation from C code to machine code (not in either
> > direction), so anything that more or less depends on that will always
> > be awkward.  If you can actually express the dependency in your source
> > code that will get us 95% to where we want to be.

^^^

> > > Data dependencies, control dependencies and address dependencies, C
> > > doesn't really like them, we rely on them. It would be awesome if we can
> > > fix this.
> > 
> > Yes.  The problem is that C is a high-level language.  All C semantics
> > are expressed on a an "as-if" level, never as "do this, then that" --
> > well, of course that *is* what it says, it's an imperative language just
> > like most, but that is just how you *think* about things on a conceptual
> > level, there is nothing that says the machine code has to do the same
> > thing in the same order as you wrote!
> 
> Which is exactly why these conversations are often difficult.  There is
> a tension between pushing the as-if rule as far as possible within the
> compiler on the one hand and allowing developers to write code that does
> what is needed on the other.  ;-)

There is a tension between what users expect from the compiler and what
actually is promised.  The compiler is not pushing the as-if rule any
further than it always has: it just becomes better at optimising over
time.  The as-if rule is and always has been absolute.

What is needed to get any progress is for user expectations to be
feasible and not contradict existing requirements.  See "^^^" above.


Segher