linux-kernel - Re: Current LKMM patch disposition

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Date:   Mon, 13 Feb 2023 19:36:42 -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

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?

Thanks,

 - Joel