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

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Date:   Tue, 8 Jun 2021 10:28:51 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alexander Monakov <amonakov@...ras.ru>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jakub Jelinek <jakub@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will@...nel.org>,
        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 Tue, Jun 08, 2021 at 01:22:58PM +0200, Peter Zijlstra wrote:
> Works for me; and note how it mirrors how we implemented volatile_if()
> in the first place, by doing an expression wrapper.
> 
> __builtin_ctrl_depends(expr) would have to:
> 
>  - ensure !__builtin_const_p(expr)	(A)

Why would it be an error if __builtin_constant_p(expr)?  In many
programs the compiler can figure out some expression does never change.
Having a control dependency on sometthing like that is not erroneous.

>  - imply an acquire compiler fence	(B)
>  - ensure cond-branch is emitted	(C)

(C) is almost impossible to do.  This should be reformulated to talk
about the effect of the generated code, instead.

> *OR*
> 
>  - ensure !__builtin_const_p(expr);		(A)
>  - upgrade the load in @expr to load-acquire	(D)

So that will only work if there is exactly one read from memory in expr?
That is problematic.

This needs some work.


Segher