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

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Date:   Fri, 1 Oct 2021 13:28:01 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Marco Elver <elver@...gle.com>, Will Deacon <will@...nel.org>,
        paulmck <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        j alglave <j.alglave@....ac.uk>,
        luc maranget <luc.maranget@...ia.fr>,
        akiyks <akiyks@...il.com>,
        linux-toolchains <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 Oct 1, 2021, at 12:20 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:
[...]
> But again - a lot of these made-up examples are exactly that: made up.
> For us to have a ctrl_dep() macro, I really want to see an actual
> honest-to-goodness case of this that we can point to.

I've spent some quality time staring at generated assembler diff in the past
days, and looking for code patterns of refcount_dec_and_test users, without
much success. There are some cases which end up working by chance, e.g. in
cases where the if leg has a smp_acquire__after_ctrl_dep and the else leg has
code that emits a barrier(), but I did not find any buggy generated
code per se. In order to observe those issues in real life, we would
really need to have identical then/else legs to the branch.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com