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.pa...@amarulasolutions.com>
> > Cc: Alan Stern <st...@rowland.harvard.edu>
> > Cc: Will Deacon <will.dea...@arm.com>
> > Cc: Peter Zijlstra <pet...@infradead.org>
> > Cc: Boqun Feng <boqun.f...@gmail.com>
> > Cc: Nicholas Piggin <npig...@gmail.com>
> > Cc: David Howells <dhowe...@redhat.com>
> > Cc: Jade Alglave <j.algl...@ucl.ac.uk>
> > Cc: Luc Maranget <luc.maran...@inria.fr>
> > Cc: "Paul E. McKenney" <paul...@linux.ibm.com>
> > Cc: Akira Yokosawa <aki...@gmail.com>
> > Cc: Daniel Lustig <dlus...@nvidia.com>
> > ---
>
> Acked-by: Alan Stern <st...@rowland.harvard.edu>
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.
> >
>
