linux-kernel - Re: [PATCH] doc: Update memory-barriers.txt for read-to-write dependencies

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Date:   Wed, 5 Jul 2017 10:46:45 +0100
From:   Will Deacon <will.deacon@....com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, ldr709@...il.com,
        dhowells@...hat.com, peterz@...radead.org, corbet@....net,
        stern@...land.harvard.edu, parri.andrea@...il.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr
Subject: Re: [PATCH] doc: Update memory-barriers.txt for read-to-write
 dependencies

On Tue, Jul 04, 2017 at 02:46:51PM -0700, Paul E. McKenney wrote:
> On Tue, Jul 04, 2017 at 05:36:36PM +0100, Will Deacon wrote:
> > On Mon, Jul 03, 2017 at 10:41:06AM -0700, Paul E. McKenney wrote:
> > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > > index 9d5e0f853f08..7be80911e502 100644
> > > --- a/Documentation/memory-barriers.txt
> > > +++ b/Documentation/memory-barriers.txt
> > > @@ -594,7 +594,23 @@ between the address load and the data load:
> > >  This enforces the occurrence of one of the two implications, and prevents the
> > >  third possibility from arising.
> > >  
> > > -A data-dependency barrier must also order against dependent writes:
> > > +
> > > +[!] Note that this extremely counterintuitive situation arises most easily on
> > > +machines with split caches, so that, for example, one cache bank processes
> > > +even-numbered cache lines and the other bank processes odd-numbered cache
> > > +lines.  The pointer P might be stored in an odd-numbered cache line, and the
> > > +variable B might be stored in an even-numbered cache line.  Then, if the
> > > +even-numbered bank of the reading CPU's cache is extremely busy while the
> > > +odd-numbered bank is idle, one can see the new value of the pointer P (&B),
> > > +but the old value of the variable B (2).
> > > +
> > > +
> > > +A data-dependency barrier is not required to order dependent writes
> > > +because the CPUs that the Linux kernel supports don't do writes until
> > > +they are certain (1) that the write will actually happen, (2) of the
> > > +location of the write, and (3) of the value to be written.  But please
> > > +carefully read the "CONTROL DEPENDENCIES" section:  The compiler can
> > > +and does break control dependencies in a great many situations.
> > 
> > This makes it sound like only control dependencies are susceptible to
> > being broken by compiler optimisations, so I'd drop the "control" and
> > just say "The compiler can and does break dependencies in a great many
> > situations".
> > 
> > With that:
> > 
> > Acked-by: Will Deacon <will.deacon@....com>
> 
> Done!  I also added a pointer to Documentation/RCU/rcu_dereference.txt,
> which lists some of the vandalism^Woptimizations that modern compilers
> can commit.  Does the updated patch below capture it?

Yes, thanks Paul.

Will