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tools/memory-model: Update the informal documentation 

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@amarulasolutions.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Daniel Lustig <dlustig@nvidia.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
alistair/sunxi64-5.4-dsi
Andrea Parri 2019-06-29 23:10:44 +02:00 committed by Paul E. McKenney
parent 6240973e56
commit 6738ff85c3
2 changed files with 27 additions and 32 deletions
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43
tools/memory-model/Documentation/explanation.txt
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@ -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.
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.
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.
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.
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').
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

16
tools/memory-model/README
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@ -167,15 +167,15 @@ scripts Various scripts, see scripts/README.
LIMITATIONS
===========
The Linux-kernel memory model has the following limitations:
The Linux-kernel memory model (LKMM) 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).
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.