linux-kernel - Re: [RFC PATCH -lkmm] docs/memory-barriers: Fix inconsistent name of 'data dependency barrier'

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Date:   Sat, 28 May 2022 09:57:19 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Akira Yokosawa <akiyks@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [RFC PATCH -lkmm] docs/memory-barriers: Fix inconsistent name of
 'data dependency barrier'

On Sat, May 28, 2022 at 01:15:30PM +0900, Akira Yokosawa wrote:
> The term "data dependency barrier", which has been in
> memory-barriers.txt ever since it was first authored by David Howells,
> has become confusing due to the fact that in LKMM's explanations.txt
> and elsewhere, "data dependency" is used mostly for load-to-store data
> dependency.
> 
> To prevent further confusions, do the following changes:
> 
>   - substitute "address-dependency barrier" for "data dependency barrier";
>   - add note on the removal of kernel APIs for explicit address-
>     dependency barriers in kernel release v5.9;
>   - add note on the section title rename;
>   - use READ_ONCE_OLD() for READ_ONCE() of pre-4.15 (no address-
>     dependency implication) in code snippets;
>   - fix number of CPU memory barrier APIs;
>   - and a few more context adjustments.
> 
> Note: Line break cleanups are deferred to a follow-up patch.
> 
> Reported-by: "Michael S. Tsirkin" <mst@...hat.com>
> Signed-off-by: Akira Yokosawa <akiyks@...il.com>
> Cc: "Paul E. McKenney" <paulmck@...ux.ibm.com>
> Cc: Alan Stern <stern@...land.harvard.edu>
> Cc: Will Deacon <will@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Andrea Parri <parri.andrea@...il.com>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Cc: David Howells <dhowells@...hat.com>
> Cc: Daniel Lustig <dlustig@...dia.com>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> Cc: Jonathan Corbet <corbet@....net>
> ---
> This is a response to Michael's report back in last November [1].
> 
> [1]: "data dependency naming inconsistency":
>      https://lore.kernel.org/r/20211011064233-mutt-send-email-mst@kernel.org/
> 
> In the thread, I suggested removing all the explanations of "data dependency
> barriers", which Paul thought was reasonable.
> 
> However, such removals would require rewriting the notoriously
> hard-to-grasp document, which I'm not quite up to.
> I have become more inclined to just substitute "address-dependency
> barrier" for "data dependency barrier" considering the fact that
> READ_ONCE() has an implicit memory barrier for Alpha.
> 
> This RFC patch is the result of such an attempt.
> 
> Note: I made a mistake in the thread above. Kernel APIs for explicit data
> dependency barriers were removed in v5.9.
> I confused the removal with the addition of the barrier to Alpha's
> READ_ONCE() in v4.15.
> 
> Any feedback is welcome!
> 
>         Thanks, Akira

This looks great!  Thanks a lot for working on it.  The way
memory-barriers.txt misuses "data dependency" to mean "address
dependency" has bothered me for a long time; I'm very glad that it
will finally get cleaned up.

Alan