linux-kernel - Re: [PATCH v2] tools: memory-model: Make plain accesses carry dependencies

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Date:   Sun, 4 Dec 2022 00:33:06 -0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Jonas Oberhauser <jonas.oberhauser@...wei.com>
Cc:     "paulmck@...nel.org" <paulmck@...nel.org>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "will@...nel.org" <will@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "akiyks@...il.com" <akiyks@...il.com>,
        "dlustig@...dia.com" <dlustig@...dia.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "urezki@...il.com" <urezki@...il.com>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "frederic@...nel.org" <frederic@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tools: memory-model: Make plain accesses carry
 dependencies

On Sun, Dec 04, 2022 at 12:15:27AM +0000, Jonas Oberhauser wrote:
> 
> 
> -----Original Message-----
> From: Paul E. McKenney [mailto:paulmck@...nel.org] 
> Sent: Sunday, December 4, 2022 12:11 AM
> To: stern@...land.harvard.edu
> > On Sat, Dec 03, 2022 at 04:32:19PM -0500, stern@...land.harvard.edu wrote:
> > > My advice: Omit them both.
> > It would be good to reference something or another.  ;-)
> 
> I also prefer to not refer to that presentation. 
> If there is a feeling that more context is needed, I would first
> prefer to enhance the commit message itself in some way. (Personally I
> don't feel that this is needed, and the imho the issue stands by
> itself even without reference to OOTA, which could be resolved fully
> independently e.g. by Viktor's suggestion to just axiomatically forbid
> OOTA --- the issue addressed by this patch would still exist). If

The reason that I'm gving you a hard time is that I haven't seen a real
world code usage that needs this fix, maybe there is one and I'm just
stupid and not knowing about. Your litmus explains the problem very well
but it's better if there is real world code expecting this ordering.

Not saying real world code is essential for memory model changes, but
without it, I guess the rationale of this patch is "plain accesses
shouldn't be weaker than registers" or "This (plain accesses don't
provide dependencies) is too conservative", and these don't seem very
strong without a bigger motivation behind it.

Also I'm in the impression that people love to put
READ_ONCE()/WRITE_ONCE() when they have some ordering issues (in real
world or with LKMM). Although I don't like this, but you cannot blame
people who just want more guarantee allowing their code to work ;-( This
is also another reason that I'd like to see strong reasoning of this
change.

Besides, could you also explain a little bit why only "data;rfi" can be
"carry-dep" but "ctrl;rfi" and "addr;rfi" cannot? I think it's because
there are special cases when compilers can figure out a condition being
true or an address being constant therefore break the dependency? But
maybe I'm wrong or missing something. Thank you!

(Please don't be mad at me, sometimes I'm just slow to understand things
;-))

Regards,
Boqun

> that's not satisfactory, I would also consent to publishing the
> e-mails from the thread starting where I relayed Viktor's observation
> of the relaxed accesses, but I don't recall it saying anything
> substantially beyond the current commit log + the documentation
> included in the patch.
> 
> best wishes, jonas