linux-yocto/tools/memory-model

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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
------------  ----------
     -- 4.14  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/README Guide to the other documents in the Documentation/ directory.

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.