linux-kernel - Re: [PATCH RFC] tools/memory-model: Adjust ctrl dependency definition

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Message-ID: <YtFrPoOARrL/etBu@rowland.harvard.edu>
Date:   Fri, 15 Jul 2022 09:27:26 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Paul Heidekrüger <Paul.Heidekrueger@...tum.de>
Cc:     clang-built-linux <llvm@...ts.linux.dev>,
        linux-toolchains@...r.kernel.org,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, Palmer Dabbelt <palmer@...belt.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Marco Elver <elver@...gle.com>,
        Charalampos Mainas <charalampos.mainas@...il.com>,
        Pramod Bhatotia <pramod.bhatotia@...tum.de>,
        Soham Chakraborty <s.s.chakraborty@...elft.nl>,
        Martin Fink <martin.fink@...tum.de>
Subject: Re: [PATCH RFC] tools/memory-model: Adjust ctrl dependency definition

On Fri, Jul 15, 2022 at 02:27:28PM +0200, Paul Heidekrüger wrote:
> I have just been thinking about how to word this patch; am I correct in
> assuming that the LKMM does not deal with loop conditions? Or in other
> words, there is no way for a loop condition to impose a ctrl dependency on
> any WRITE_ONCE's in the loop body? It are only if and switch statements the
> LKMM is concerned with in the case of ctrl dependencies?

In theory, the LKMM does say that a loop condition imposes a control 
dependency on any memory accesses within the loop body.  However, the 
herd7 tool has only very limited support for looping constructs, so in 
practice it's not possible to create suitable litmus tests with loops.

Alan