linux-kernel - Re: [PATCH memory-model 5/8] tools/memory-model: Add a glossary of LKMM terms

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Date:   Fri, 6 Nov 2020 11:59:12 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        kernel-team@...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: [PATCH memory-model 5/8] tools/memory-model: Add a glossary of
 LKMM terms

On Fri, Nov 06, 2020 at 02:23:51PM -0500, Alan Stern wrote:
> On Fri, Nov 06, 2020 at 10:04:46AM -0800, Paul E. McKenney wrote:
> > On Fri, Nov 06, 2020 at 11:59:30AM -0500, Alan Stern wrote:
> > > > +	 See also "Control Dependency".
> > > 
> > > There should also be an entry for "Data Dependency", linked from here
> > > and from Control Dependency.
> > > 
> > > > +Marked Access:  An access to a variable that uses an special function or
> > > > +	macro such as "r1 = READ_ONCE()" or "smp_store_release(&a, 1)".
> > > 
> > > How about "r1 = READ_ONCE(x)"?
> > 
> > Good catches!  I am planning to squash the commit below into the
> > original.  Does that cover it?
> 
> No, because you didn't add a glossary entry for "Data Dependency" and 
> there's no link from "Control Dependency" to "Data Dependency".

Sigh.  I was thinking "entry in the list", and didn't even thing to
check for an entry in the glossary as a whole.  With the patch below
(on top of the one sent earlier), are we good?

							Thanx, Paul

------------------------------------------------------------------------

commit 5a49c32551e83d30e304d6c3fbb660737ba2654e
Author: Paul E. McKenney <paulmck@...nel.org>
Date:   Fri Nov 6 11:57:25 2020 -0800

    fixup! tools/memory-model: Add a glossary of LKMM terms
    
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/tools/memory-model/Documentation/glossary.txt b/tools/memory-model/Documentation/glossary.txt
index 471bf13..b2da636 100644
--- a/tools/memory-model/Documentation/glossary.txt
+++ b/tools/memory-model/Documentation/glossary.txt
@@ -64,7 +64,7 @@ Control Dependency:  When a later store's execution depends on a test
 	 fragile, and can be easily destroyed by optimizing compilers.
 	 Please see control-dependencies.txt for more information.
 
-	 See also "Address Dependency".
+	 See also "Address Dependency" and "Data Dependency".
 
 Cycle:	Memory-barrier pairing is restricted to a pair of CPUs, as the
 	name suggests.	And in a great many cases, a pair of CPUs is all
@@ -85,6 +85,23 @@ Cycle:	Memory-barrier pairing is restricted to a pair of CPUs, as the
 
 	See also "Pairing".
 
+Data Dependency:  When the data written by a later store is computed based
+	on the value returned by an earlier load, a "data dependency"
+	extends from that load to that later store.  For example:
+
+	 1 r1 = READ_ONCE(x);
+	 2 WRITE_ONCE(y, r1 + 1);
+
+	In this case, the data dependency extends from the READ_ONCE()
+	on line 1 to the WRITE_ONCE() on line 2.  Data dependencies are
+	fragile and can be easily destroyed by optimizing compilers.
+	Because optimizing compilers put a great deal of effort into
+	working out what values integer variables might have, this is
+	especially true in cases where the dependency is carried through
+	an integer.
+
+	See also "Address Dependency" and "Control Dependency".
+
 From-Reads (fr):  When one CPU's store to a given variable happened
 	too late to affect the value returned by another CPU's
 	load from that same variable, there is said to be a from-reads