Re: [PATCH] tools/memory-model: Update the informal documentation

[Paul E. McKenney]

On Mon, Jul 01, 2019 at 10:12:45AM -0400, Alan Stern wrote:
> On Sat, 29 Jun 2019, Andrea Parri wrote:
> 
> > The formal memory consistency model has added support for plain accesses
> > (and data races).  While updating the informal documentation to describe
> > this addition to the model is highly desirable and important future work,
> > update the informal documentation to at least acknowledge such addition.
> > 
> > Signed-off-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> > Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> > Cc: David Howells <dhowells@xxxxxxxxxx>
> > Cc: Jade Alglave <j.alglave@xxxxxxxxx>
> > Cc: Luc Maranget <luc.maranget@xxxxxxxx>
> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx>
> > Cc: Akira Yokosawa <akiyks@xxxxxxxxx>
> > Cc: Daniel Lustig <dlustig@xxxxxxxxxx>
> > ---
> 
> Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>

Applied, thank you both!

							Thanx, Paul

> >  tools/memory-model/Documentation/explanation.txt | 47 +++++++++++-------------
> >  tools/memory-model/README                        | 18 ++++-----
> >  2 files changed, 30 insertions(+), 35 deletions(-)
> > 
> > diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
> > index 68caa9a976d0c..b42f7cd718242 100644
> > --- a/tools/memory-model/Documentation/explanation.txt
> > +++ b/tools/memory-model/Documentation/explanation.txt
> > @@ -42,7 +42,8 @@ linux-kernel.bell and linux-kernel.cat files that make up the formal
> >  version of the model; they are extremely terse and their meanings are
> >  far from clear.
> >  
> > -This document describes the ideas underlying the LKMM.  It is meant
> > +This document describes the ideas underlying the LKMM, but excluding
> > +the modeling of bare C (or plain) shared memory accesses.  It is meant
> >  for people who want to understand how the model was designed.  It does
> >  not go into the details of the code in the .bell and .cat files;
> >  rather, it explains in English what the code expresses symbolically.
> > @@ -354,31 +355,25 @@ be extremely complex.
> >  Optimizing compilers have great freedom in the way they translate
> >  source code to object code.  They are allowed to apply transformations
> >  that add memory accesses, eliminate accesses, combine them, split them
> > -into pieces, or move them around.  Faced with all these possibilities,
> > -the LKMM basically gives up.  It insists that the code it analyzes
> > -must contain no ordinary accesses to shared memory; all accesses must
> > -be performed using READ_ONCE(), WRITE_ONCE(), or one of the other
> > -atomic or synchronization primitives.  These primitives prevent a
> > -large number of compiler optimizations.  In particular, it is
> > -guaranteed that the compiler will not remove such accesses from the
> > -generated code (unless it can prove the accesses will never be
> > -executed), it will not change the order in which they occur in the
> > -code (within limits imposed by the C standard), and it will not
> > -introduce extraneous accesses.
> > -
> > -This explains why the MP and SB examples above used READ_ONCE() and
> > -WRITE_ONCE() rather than ordinary memory accesses.  Thanks to this
> > -usage, we can be certain that in the MP example, P0's write event to
> > -buf really is po-before its write event to flag, and similarly for the
> > -other shared memory accesses in the examples.
> > -
> > -Private variables are not subject to this restriction.  Since they are
> > -not shared between CPUs, they can be accessed normally without
> > -READ_ONCE() or WRITE_ONCE(), and there will be no ill effects.  In
> > -fact, they need not even be stored in normal memory at all -- in
> > -principle a private variable could be stored in a CPU register (hence
> > -the convention that these variables have names starting with the
> > -letter 'r').
> > +into pieces, or move them around.  The use of READ_ONCE(), WRITE_ONCE(),
> > +or one of the other atomic or synchronization primitives prevents a
> > +large number of compiler optimizations.  In particular, it is guaranteed
> > +that the compiler will not remove such accesses from the generated code
> > +(unless it can prove the accesses will never be executed), it will not
> > +change the order in which they occur in the code (within limits imposed
> > +by the C standard), and it will not introduce extraneous accesses.
> > +
> > +The MP and SB examples above used READ_ONCE() and WRITE_ONCE() rather
> > +than ordinary memory accesses.  Thanks to this usage, we can be certain
> > +that in the MP example, the compiler won't reorder P0's write event to
> > +buf and P0's write event to flag, and similarly for the other shared
> > +memory accesses in the examples.
> > +
> > +Since private variables are not shared between CPUs, they can be
> > +accessed normally without READ_ONCE() or WRITE_ONCE().  In fact, they
> > +need not even be stored in normal memory at all -- in principle a
> > +private variable could be stored in a CPU register (hence the convention
> > +that these variables have names starting with the letter 'r').
> >  
> >  
> >  A WARNING
> > diff --git a/tools/memory-model/README b/tools/memory-model/README
> > index 2b87f3971548c..fc07b52f20286 100644
> > --- a/tools/memory-model/README
> > +++ b/tools/memory-model/README
> > @@ -167,15 +167,15 @@ scripts	Various scripts, see scripts/README.
> >  LIMITATIONS
> >  ===========
> >  
> > -The Linux-kernel memory model has the following limitations:
> > -
> > -1.	Compiler optimizations are not modeled.  Of course, the use
> > -	of READ_ONCE() and WRITE_ONCE() limits the compiler's ability
> > -	to optimize, but there is Linux-kernel code that uses bare C
> > -	memory accesses.  Handling this code is on the to-do list.
> > -	For more information, see Documentation/explanation.txt (in
> > -	particular, the "THE PROGRAM ORDER RELATION: po AND po-loc"
> > -	and "A WARNING" sections).
> > +The Linux-kernel memory model (LKMM) has the following limitations:
> > +
> > +1.	Compiler optimizations are not accurately modeled.  Of course,
> > +	the use of READ_ONCE() and WRITE_ONCE() limits the compiler's
> > +	ability to optimize, but under some circumstances it is possible
> > +	for the compiler to undermine the memory model.  For more
> > +	information, see Documentation/explanation.txt (in particular,
> > +	the "THE PROGRAM ORDER RELATION: po AND po-loc" and "A WARNING"
> > +	sections).
> >  
> >  	Note that this limitation in turn limits LKMM's ability to
> >  	accurately model address, control, and data dependencies.
> > 
>