WSL2-Linux-Kernel/tools/memory-model/Documentation
Jonas Oberhauser 9ba7d3b3b8 tools: memory-model: Make plain accesses carry dependencies
As reported by Viktor, plain accesses in LKMM are weaker than
accesses to registers: the latter carry dependencies but the former
do not. This is exemplified in the following snippet:

  int r = READ_ONCE(*x);
  WRITE_ONCE(*y, r);

Here a data dependency links the READ_ONCE() to the WRITE_ONCE(),
preserving their order, because the model treats r as a register.
If r is turned into a memory location accessed by plain accesses,
however, the link is broken and the order between READ_ONCE() and
WRITE_ONCE() is no longer preserved.

This is too conservative, since any optimizations on plain
accesses that might break dependencies are also possible on
registers; it also contradicts the intuitive notion of "dependency"
as the data stored by the WRITE_ONCE() does depend on the data read
by the READ_ONCE(), independently of whether r is a register or a
memory location.

This is resolved by redefining all dependencies to include
dependencies carried by memory accesses; a dependency is said to be
carried by memory accesses (in the model: carry-dep) from one load
to another load if the initial load is followed by an arbitrarily
long sequence alternating between stores and loads of the same
thread, where the data of each store depends on the previous load,
and is read by the next load.

Any dependency linking the final load in the sequence to another
access also links the initial load in the sequence to that access.

More deep details can be found in this LKML discussion:

https://lore.kernel.org/lkml/d86295788ad14a02874ab030ddb8a6f8@huawei.com/

Reported-by: Viktor Vafeiadis <viktor@mpi-sws.org>
Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huawei.com>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2023-01-03 20:47:04 -08:00
..
README tools/memory-model: Document categories of ordering primitives 2020-11-06 17:24:50 -08:00
access-marking.txt tools/memory-model: Document data_race(READ_ONCE()) 2021-07-27 11:48:55 -07:00
cheatsheet.txt tools/memory-model: Expand the cheatsheet.txt notion of relaxed 2020-09-04 11:58:15 -07:00
control-dependencies.txt tools/memory-model: Document categories of ordering primitives 2020-11-06 17:24:50 -08:00
explanation.txt tools: memory-model: Make plain accesses carry dependencies 2023-01-03 20:47:04 -08:00
glossary.txt doc: Update rcu_dereference.rst reference 2021-03-08 14:29:22 -08:00
litmus-tests.txt tools/memory-model: Clarify LKMM's limitations in litmus-tests.txt 2022-08-31 05:15:31 -07:00
ordering.txt tools/memory-model: Document categories of ordering primitives 2020-11-06 17:24:50 -08:00
recipes.txt tools/memory-model: Update recipes.txt prime_numbers.c path 2020-09-03 09:51:00 -07:00
references.txt Replace HTTP links with HTTPS ones: LKMM 2020-09-03 09:51:00 -07:00
simple.txt tools/memory-model: Remove reference to atomic_ops.rst 2021-03-08 14:29:22 -08:00

README

It has been said that successful communication requires first identifying
what your audience knows and then building a bridge from their current
knowledge to what they need to know.  Unfortunately, the expected
Linux-kernel memory model (LKMM) audience might be anywhere from novice
to expert both in kernel hacking and in understanding LKMM.

This document therefore points out a number of places to start reading,
depending on what you know and what you would like to learn.  Please note
that the documents later in this list assume that the reader understands
the material provided by documents earlier in this list.

o	You are new to Linux-kernel concurrency: simple.txt

o	You have some background in Linux-kernel concurrency, and would
	like an overview of the types of low-level concurrency primitives
	that the Linux kernel provides:  ordering.txt

	Here, "low level" means atomic operations to single variables.

o	You are familiar with the Linux-kernel concurrency primitives
	that you need, and just want to get started with LKMM litmus
	tests:  litmus-tests.txt

o	You are familiar with Linux-kernel concurrency, and would
	like a detailed intuitive understanding of LKMM, including
	situations involving more than two threads:  recipes.txt

o	You would like a detailed understanding of what your compiler can
	and cannot do to control dependencies:  control-dependencies.txt

o	You are familiar with Linux-kernel concurrency and the use of
	LKMM, and would like a quick reference:  cheatsheet.txt

o	You are familiar with Linux-kernel concurrency and the use
	of LKMM, and would like to learn about LKMM's requirements,
	rationale, and implementation:	explanation.txt

o	You are interested in the publications related to LKMM, including
	hardware manuals, academic literature, standards-committee
	working papers, and LWN articles:  references.txt


====================
DESCRIPTION OF FILES
====================

README
	This file.

cheatsheet.txt
	Quick-reference guide to the Linux-kernel memory model.

control-dependencies.txt
	Guide to preventing compiler optimizations from destroying
	your control dependencies.

explanation.txt
	Detailed description of the memory model.

litmus-tests.txt
	The format, features, capabilities, and limitations of the litmus
	tests that LKMM can evaluate.

ordering.txt
	Overview of the Linux kernel's low-level memory-ordering
	primitives by category.

recipes.txt
	Common memory-ordering patterns.

references.txt
	Background information.

simple.txt
	Starting point for someone new to Linux-kernel concurrency.
	And also a reminder of the simpler approaches to concurrency!