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LINUX KERNEL MEMORY CONSISTENCY MODEL
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============
INTRODUCTION
============
This directory contains the memory consistency model (memory model, for
short) of the Linux kernel, written in the "cat" language and executable
by the externally provided "herd7" simulator, which exhaustively explores
the state space of small litmus tests.
In addition, the "klitmus7" tool (also externally provided) may be used
to convert a litmus test to a Linux kernel module, which in turn allows
that litmus test to be exercised within the Linux kernel.
============
REQUIREMENTS
============
Version 7.52 or higher of the "herd7" and "klitmus7" tools must be
downloaded separately:
https://github.com/herd/herdtools7
See "herdtools7/INSTALL.md" for installation instructions.
Note that although these tools usually provide backwards compatibility,
this is not absolutely guaranteed.
For example, a future version of herd7 might not work with the model
in this release. A compatible model will likely be made available in
a later release of Linux kernel.
If you absolutely need to run the model in this particular release,
please try using the exact version called out above.
klitmus7 is independent of the model provided here. It has its own
dependency on a target kernel release where converted code is built
and executed. Any change in kernel APIs essential to klitmus7 will
necessitate an upgrade of klitmus7.
If you find any compatibility issues in klitmus7, please inform the
memory model maintainers.
klitmus7 Compatibility Table
----------------------------
============ ==========
target Linux herdtools7
------------ ----------
-- 4.18 7.48 --
4.15 -- 4.19 7.49 --
4.20 -- 5.5 7.54 --
5.6 -- 7.56 --
============ ==========
==================
BASIC USAGE: HERD7
==================
The memory model is used, in conjunction with "herd7", to exhaustively
explore the state space of small litmus tests. Documentation describing
the format, features, capabilities and limitations of these litmus
tests is available in tools/memory-model/Documentation/litmus-tests.txt.
Example litmus tests may be found in the Linux-kernel source tree:
tools/memory-model/litmus-tests/
Documentation/litmus-tests/
Several thousand more example litmus tests are available here:
https://github.com/paulmckrcu/litmus
https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/herd
https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/litmus
Documentation describing litmus tests and now to use them may be found
here:
tools/memory-model/Documentation/litmus-tests.txt
The remainder of this section uses the SB+fencembonceonces.litmus test
located in the tools/memory-model directory.
To run SB+fencembonceonces.litmus against the memory model:
$ cd $LINUX_SOURCE_TREE/tools/memory-model
$ herd7 -conf linux-kernel.cfg litmus-tests/SB+fencembonceonces.litmus
Here is the corresponding output:
Test SB+fencembonceonces Allowed
States 3
0:r0=0; 1:r0=1;
0:r0=1; 1:r0=0;
0:r0=1; 1:r0=1;
No
Witnesses
Positive: 0 Negative: 3
Condition exists (0:r0=0 /\ 1:r0=0)
Observation SB+fencembonceonces Never 0 3
Time SB+fencembonceonces 0.01
Hash=d66d99523e2cac6b06e66f4c995ebb48
The "Positive: 0 Negative: 3" and the "Never 0 3" each indicate that
this litmus test's "exists" clause can not be satisfied.
See "herd7 -help" or "herdtools7/doc/" for more information on running the
tool itself, but please be aware that this documentation is intended for
people who work on the memory model itself, that is, people making changes
to the tools/memory-model/linux-kernel.* files. It is not intended for
people focusing on writing, understanding, and running LKMM litmus tests.
=====================
BASIC USAGE: KLITMUS7
=====================
The "klitmus7" tool converts a litmus test into a Linux kernel module,
which may then be loaded and run.
For example, to run SB+fencembonceonces.litmus against hardware:
$ mkdir mymodules
$ klitmus7 -o mymodules litmus-tests/SB+fencembonceonces.litmus
$ cd mymodules ; make
$ sudo sh run.sh
The corresponding output includes:
Test SB+fencembonceonces Allowed
Histogram (3 states)
644580 :>0:r0=1; 1:r0=0;
644328 :>0:r0=0; 1:r0=1;
711092 :>0:r0=1; 1:r0=1;
No
Witnesses
Positive: 0, Negative: 2000000
Condition exists (0:r0=0 /\ 1:r0=0) is NOT validated
Hash=d66d99523e2cac6b06e66f4c995ebb48
Observation SB+fencembonceonces Never 0 2000000
Time SB+fencembonceonces 0.16
The "Positive: 0 Negative: 2000000" and the "Never 0 2000000" indicate
that during two million trials, the state specified in this litmus
test's "exists" clause was not reached.
And, as with "herd7", please see "klitmus7 -help" or "herdtools7/doc/"
for more information. And again, please be aware that this documentation
is intended for people who work on the memory model itself, that is,
people making changes to the tools/memory-model/linux-kernel.* files.
It is not intended for people focusing on writing, understanding, and
running LKMM litmus tests.
====================
DESCRIPTION OF FILES
====================
Documentation/cheatsheet.txt
Quick-reference guide to the Linux-kernel memory model.
Documentation/explanation.txt
Describes the memory model in detail.
Documentation/litmus-tests.txt
Describes the format, features, capabilities, and limitations
of the litmus tests that LKMM can evaluate.
Documentation/recipes.txt
Lists common memory-ordering patterns.
Documentation/references.txt
Provides background reading.
Documentation/simple.txt
Starting point for someone new to Linux-kernel concurrency.
And also for those needing a reminder of the simpler approaches
to concurrency!
linux-kernel.bell
Categorizes the relevant instructions, including memory
references, memory barriers, atomic read-modify-write operations,
lock acquisition/release, and RCU operations.
More formally, this file (1) lists the subtypes of the various
event types used by the memory model and (2) performs RCU
read-side critical section nesting analysis.
linux-kernel.cat
Specifies what reorderings are forbidden by memory references,
memory barriers, atomic read-modify-write operations, and RCU.
More formally, this file specifies what executions are forbidden
by the memory model. Allowed executions are those which
satisfy the model's "coherence", "atomic", "happens-before",
"propagation", and "rcu" axioms, which are defined in the file.
linux-kernel.cfg
Convenience file that gathers the common-case herd7 command-line
arguments.
linux-kernel.def
Maps from C-like syntax to herd7's internal litmus-test
instruction-set architecture.
litmus-tests
Directory containing a few representative litmus tests, which
are listed in litmus-tests/README. A great deal more litmus
tests are available at https://github.com/paulmckrcu/litmus.
lock.cat
Provides a front-end analysis of lock acquisition and release,
for example, associating a lock acquisition with the preceding
and following releases and checking for self-deadlock.
More formally, this file defines a performance-enhanced scheme
for generation of the possible reads-from and coherence order
relations on the locking primitives.
README
This file.
scripts Various scripts, see scripts/README.
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