linux-kernel - Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency

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Message-ID: <CAHk-=wjd_BJiJYZ99PAoc4mQ3QTiZrt-tRdznj3g9kU8-gYsAA@mail.gmail.com>
Date:   Wed, 29 Sep 2021 07:54:14 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.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-toolchains@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency

On Wed, Sep 29, 2021 at 7:47 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> And if there *is* need ("look, we have that same store in both the if-
> and the else-statement" or whatever), then say so, and state that
> thing.

Side note: I'd also like the commit that introduces this to talk very
explicitly about the particular case that it is used doe and that it
fixes. No "this can happen". A "this happened, here's the _actual_
wrong code generation, and look how this new ctrl_dep() macro fixes
it".

When it's this subtle, I don't want theoretical arguments. I want
actual outstanding and real bugs.

Because I get the feeling that there were very few actual realistic
examples of this, only made-up theoretical cases that wouldn't ever
really be found in real code.

                Linus