On Sat, Jun 29, 2019 at 11:10:44PM +0200, 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>
Queued for review, thank you, Andrea!
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.
> --
> 2.7.4
>
