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

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Message-ID: <20201106180445.GX3249@paulmck-ThinkPad-P72>
Date:   Fri, 6 Nov 2020 10:04:46 -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 11:59:30AM -0500, Alan Stern wrote:
> On Thu, Nov 05, 2020 at 02:00:14PM -0800, paulmck@...nel.org wrote:
> > From: "Paul E. McKenney" <paulmck@...nel.org>
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> >  tools/memory-model/Documentation/glossary.txt | 155 ++++++++++++++++++++++++++
> >  1 file changed, 155 insertions(+)
> >  create mode 100644 tools/memory-model/Documentation/glossary.txt
> > 
> > diff --git a/tools/memory-model/Documentation/glossary.txt b/tools/memory-model/Documentation/glossary.txt
> > new file mode 100644
> > index 0000000..036fa28
> > --- /dev/null
> > +++ b/tools/memory-model/Documentation/glossary.txt
> > @@ -0,0 +1,155 @@
> > +This document contains brief definitions of LKMM-related terms.  Like most
> > +glossaries, it is not intended to be read front to back (except perhaps
> > +as a way of confirming a diagnosis of OCD), but rather to be searched
> > +for specific terms.
> > +
> > +
> > +Address Dependency:  When the address of a later memory access is computed
> > +	based on the value returned by an earlier load, an "address
> > +	dependency" extends from that load extending to the later access.
> > +	Address dependencies are quite common in RCU read-side critical
> > +	sections:
> > +
> > +	 1 rcu_read_lock();
> > +	 2 p = rcu_dereference(gp);
> > +	 3 do_something(p->a);
> > +	 4 rcu_read_unlock();
> > +
> > +	 In this case, because the address of "p->a" on line 3 is computed
> > +	 from the value returned by the rcu_dereference() on line 2, the
> > +	 address dependency extends from that rcu_dereference() to that
> > +	 "p->a".  In rare cases, optimizing compilers can destroy address
> > +	 dependencies.	Please see Documentation/RCU/rcu_dereference.txt
> > +	 for more information.
> > +
> > +	 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?

							Thanx, Paul

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

commit 27c694f5a049d3edac1f258b888d02650cec936a
Author: Paul E. McKenney <paulmck@...nel.org>
Date:   Fri Nov 6 10:02:41 2020 -0800

    squash! tools/memory-model: Add a glossary of LKMM terms
    
    [ paulmck: Apply Alan Stern feedback. ]
    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 383151b..471bf13 100644
--- a/tools/memory-model/Documentation/glossary.txt
+++ b/tools/memory-model/Documentation/glossary.txt
@@ -22,7 +22,7 @@ Address Dependency:  When the address of a later memory access is computed
 	 dependencies.	Please see Documentation/RCU/rcu_dereference.txt
 	 for more information.
 
-	 See also "Control Dependency".
+	 See also "Control Dependency" and "Data Dependency".
 
 Acquire:  With respect to a lock, acquiring that lock, for example,
 	using spin_lock().  With respect to a non-lock shared variable,
@@ -109,7 +109,7 @@ Happens-Before (hb): A relation between two accesses in which LKMM
 	section of explanation.txt.
 
 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)".
+	macro such as "r1 = READ_ONCE(x)" or "smp_store_release(&a, 1)".
 
 	See also "Unmarked Access".