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123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224 ===================================== LINUX KERNEL MEMORY CONSISTENCY MODEL =====================================
============INTRODUCTION============
This directory contains the memory consistency model (memory model, forshort) of the Linux kernel, written in the "cat" language and executableby the externally provided "herd7" simulator, which exhaustively exploresthe state space of small litmus tests.
In addition, the "klitmus7" tool (also externally provided) may be usedto convert a litmus test to a Linux kernel module, which in turn allowsthat litmus test to be exercised within the Linux kernel.
============REQUIREMENTS============
Version 7.58 or higher of the "herd7" and "klitmus7" tools must bedownloaded 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 modelin this release. A compatible model will likely be made available ina 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 owndependency on a target kernel release where converted code is builtand executed. Any change in kernel APIs essential to klitmus7 willnecessitate an upgrade of klitmus7.
If you find any compatibility issues in klitmus7, please inform thememory 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 -- 5.16 7.56 -- 5.17 -- 7.56.1 -- ============ ==========
==================BASIC USAGE: HERD7==================
The memory model is used, in conjunction with "herd7", to exhaustivelyexplore the state space of small litmus tests. Documentation describingthe format, features, capabilities and limitations of these litmustests 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 how to use them may be foundhere:
tools/memory-model/Documentation/litmus-tests.txt
The remainder of this section uses the SB+fencembonceonces.litmus testlocated 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 thatthis litmus test's "exists" clause can not be satisfied.
See "herd7 -help" or "herdtools7/doc/" for more information on running thetool itself, but please be aware that this documentation is intended forpeople who work on the memory model itself, that is, people making changesto the tools/memory-model/linux-kernel.* files. It is not intended forpeople 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" indicatethat during two million trials, the state specified in this litmustest'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 documentationis 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, andrunning 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.
By "representative", it means the one in the litmus-tests directory is:
1) simple, the number of threads should be relatively small and each thread function should be relatively simple. 2) orthogonal, there should be no two litmus tests describing the same aspect of the memory model. 3) textbook, developers can easily copy-paste-modify the litmus tests to use the patterns on their own code.
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.