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

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Date:   Mon, 18 Jul 2022 10:24:19 +0200
From:   Paul Heidekrüger <Paul.Heidekrueger@...tum.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        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>,
        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

Paul E. McKenney <paulmck@...nel.org> wrote:

> On Fri, Jul 15, 2022 at 09:27:26AM -0400, Alan Stern wrote:
>> 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.
> 
> And Alan isn't joking.  The closest simulation that I know of is to
> combine limited loop unrolling with the "filter" clause.  The point of
> the filter clause is to eliminate from consideration executions that
> need the more iterations of the loop to be unrolled.
> 
> And that means that as far as LKMM is concerned, loop-based control
> dependencies are similar to those for nested "if" statements.
> 
> 							Thanx, Paul

Makes sense, thank you both!

Paul