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

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Date:   Thu, 9 Jun 2022 16:07:04 +0100
From:   Will Deacon <will@...nel.org>
To:     Akira Yokosawa <akiyks@...il.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        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 Wed, Jun 08, 2022 at 11:00:46AM +0900, Akira Yokosawa wrote:
> On Tue, 7 Jun 2022 10:34:08 -0400, Alan Stern wrote:
> > On Tue, Jun 07, 2022 at 02:34:33PM +0100, Will Deacon wrote:
> >> Alternatively, maybe we should be removing the historical stuff from the
> >> document altogether if it's no longer needed. We don't have any occurrences
> >> of read_barrier_depends() anymore, so why confuse people with it?
> > 
> > How about relegating discussion of these barriers to a special 
> > "historical" or "niche architecture" section of the document?  In a 
> > separate patch, of course.
> 
> Another option would be to add a section on "Ordering guarantees by
> dependencies", and explain three variants: address, data, and
> control.  We have only "control dependencies" section at the moment
> and uses "data dependency" without properly defining it. 
> 
> Address dependencies are special in that they can provide load-to-load
> ordering guarantees as well as those of load-to-store.
> Alpha doesn't provide these guarantees at the architecture level, hence
> the implicit address-dependency barrier in READ_ONCE().
> 
> Also, IIUC, arm64's READ_ONCE() is promoted to acquire-load when
> LTO is enabled.  It is to cope with the possible (but not observed
> yet) transformation of address dependencies into control dependencies,
> which can't provide load-to-load ordering guarantees.
> 
> These points can be added later in memory-barriers.txt, but I'm
> afraid I might not be up to such involved changes.

I think we should try hard to avoid documenting how specific architectures
implement the memory model (in this document, at least); it's likely to
stagnate and it encourages people to rely on the behaviours of a specific
architecture rather than the portable guarantees offered by the higher-level
memory model.

Will