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Date:   Tue, 8 Jun 2021 10:28:51 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Marco Elver <elver@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Alexander Monakov <amonakov@ispras.ru>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jakub Jelinek <jakub@redhat.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Will Deacon <will@kernel.org>,
        Andrea Parri <parri.andrea@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nick Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-toolchains@vger.kernel.org,
        linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()
Message-ID: <20210608152851.GX18427@gate.crashing.org>
References: <20210606185922.GF7746@tucnak> <CAHk-=wis8zq3WrEupCY6wcBeW3bB0WMOzaUkXpb-CsKuxM=6-w@mail.gmail.com> <alpine.LNX.2.20.13.2106070017070.7184@monopod.intra.ispras.ru> <CAHk-=wjwXs5+SOZGTaZ0bP9nsoA+PymAcGE4CBDVX3edGUcVRg@mail.gmail.com> <alpine.LNX.2.20.13.2106070956310.7184@monopod.intra.ispras.ru> <CANpmjNMwq6ENUtBunP-rw9ZSrJvZnQw18rQ47U3JuqPEQZsaXA@mail.gmail.com> <20210607152806.GS4397@paulmck-ThinkPad-P17-Gen-1> <YL5Risa6sFgnvvnG@elver.google.com> <CANpmjNNtDX+eBEpuP9-NgT6RAwHK5OgbQHT9b+8LZQJtwWpvPg@mail.gmail.com> <YL9TEqealhxBBhoS@hirez.programming.kicks-ass.net>
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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