Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Content-Type: text/plain;
        charset=utf-8
Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.100.31\))
Subject: Re: [PATCH RFC] tools/memory-model: Adjust ctrl dependency definition
From:   =?utf-8?Q?Paul_Heidekr=C3=BCger?= <Paul.Heidekrueger@in.tum.de>
In-Reply-To: <20220715152145.GZ1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Mon, 18 Jul 2022 10:24:19 +0200
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        clang-built-linux <llvm@lists.linux.dev>,
        linux-toolchains@vger.kernel.org,
        Andrea Parri <parri.andrea@gmail.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas 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>,
        Daniel Lustig <dlustig@nvidia.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, Palmer Dabbelt <palmer@dabbelt.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Marco Elver <elver@google.com>,
        Charalampos Mainas <charalampos.mainas@gmail.com>,
        Pramod Bhatotia <pramod.bhatotia@in.tum.de>,
        Soham Chakraborty <s.s.chakraborty@tudelft.nl>,
        Martin Fink <martin.fink@in.tum.de>
Content-Transfer-Encoding: quoted-printable
Message-Id: <BD651F14-7C88-4553-A4E8-986042271CAD@in.tum.de>
References: <20220615114330.2573952-1-paul.heidekrueger@in.tum.de>
 <YqnpshlsAHg7Uf9G@rowland.harvard.edu>
 <50B9D7C1-B11D-4583-9814-BFFF2C80D8CA@in.tum.de>
 <YrHUkfDWsexHRUKj@rowland.harvard.edu>
 <04B4DBD6-1262-4905-9E85-9466FC104895@in.tum.de>
 <YrnFCSjESpeQdciv@rowland.harvard.edu>
 <20F4C097-24B4-416B-95EE-AC11F5952B44@in.tum.de>
 <YtFrPoOARrL/etBu@rowland.harvard.edu>
 <20220715152145.GZ1790663@paulmck-ThinkPad-P17-Gen-1>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Precedence: bulk

Paul E. McKenney <paulmck@kernel.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 Heidekr=C3=BCger =
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?
>>=20
>> In theory, the LKMM does say that a loop condition imposes a control=20=

>> dependency on any memory accesses within the loop body.  However, the=20=

>> herd7 tool has only very limited support for looping constructs, so =
in=20
>> practice it's not possible to create suitable litmus tests with =
loops.
>=20
> 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.
>=20
> And that means that as far as LKMM is concerned, loop-based control
> dependencies are similar to those for nested "if" statements.
>=20
> 							Thanx, Paul

Makes sense, thank you both!

Paul=