linux-kernel - Re: Current LKMM patch disposition

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Date:   Mon, 13 Feb 2023 21:12:34 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        kernel-team@...a.com, mingo@...nel.org, parri.andrea@...il.com,
        will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com
Subject: Re: Current LKMM patch disposition

On Mon, Feb 13, 2023 at 8:57 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Mon, Feb 13, 2023 at 07:36:42PM -0500, Joel Fernandes wrote:
> > Thanks, I agree with most of your last email, just replying to one thing:
> >
> > > > ->rf does because of data flow causality, ->ppo does because of
> > > > program structure, so that makes sense to be ->hb.
> > > >
> > > > IMHO, ->rfi should as well, because it is embodying a flow of data, so
> > > > that is a bit confusing. It would be great to clarify more perhaps
> > > > with an example about why ->rfi cannot be ->hb, in the
> > > > "happens-before" section.
> > >
> > > Maybe.  We do talk about store forwarding, and in fact the ppo section
> > > already says:
> > >
> > > ------------------------------------------------------------------------
> > >         R ->dep W ->rfi R',
> > >
> > > where the dep link can be either an address or a data dependency.  In
> > > this situation we know it is possible for the CPU to execute R' before
> > > W, because it can forward the value that W will store to R'.
> > > ------------------------------------------------------------------------
> >
> > Thank you for pointing this out! In the text that follows this, in
> > this paragraph:
> >
> > <quote>
> > where the dep link can be either an address or a data dependency.  In
> > this situation we know it is possible for the CPU to execute R' before
> > W, because it can forward the value that W will store to R'.  But it
> > cannot execute R' before R, because it cannot forward the value before
> > it knows what that value is, or that W and R' do access the same
> > location.
> > </quote>
> >
> > The "in this situation" should be clarified that the "situation" is a
> > data-dependency. Only in the case of data-dependency,  the ->rfi
> > cannot cause misordering if I understand it correctly. However, that
> > sentence does not mention data-dependency explicitly. Or let me know
> > if I missed something?
>
> The text explicitly says that the dep link can be either an address or a
> data dependency.  In either case, R' cannot be reordered before R.
>
> In theory this doesn't have to be true for address dependencies, because
> the CPU might realize that W and R' access the same address without
> knowing what that address is.  However, I've been reliably informed that
> no existing architectures do this sort of optimization.
>
> The case of a control dependency is different, because the CPU can
> speculate that W will be executed and can speculatively forward the
> value from W to R' before it knows what value R will read.
>

Sorry, I misread it. You are right. Got it now, Thanks.

 - Joel