1 =====================================
2 LINUX KERNEL MEMORY CONSISTENCY MODEL
3 =====================================
4
5============
6INTRODUCTION
7============
8
9This directory contains the memory consistency model (memory model, for
10short) of the Linux kernel, written in the "cat" language and executable
11by the externally provided "herd7" simulator, which exhaustively explores
12the state space of small litmus tests.
13
14In addition, the "klitmus7" tool (also externally provided) may be used
15to convert a litmus test to a Linux kernel module, which in turn allows
16that litmus test to be exercised within the Linux kernel.
17
18
19============
20REQUIREMENTS
21============
22
23Version 7.52 or higher of the "herd7" and "klitmus7" tools must be
24downloaded separately:
25
26 https://github.com/herd/herdtools7
27
28See "herdtools7/INSTALL.md" for installation instructions.
29
30Note that although these tools usually provide backwards compatibility,
31this is not absolutely guaranteed.
32
33For example, a future version of herd7 might not work with the model
34in this release. A compatible model will likely be made available in
35a later release of Linux kernel.
36
37If you absolutely need to run the model in this particular release,
38please try using the exact version called out above.
39
40klitmus7 is independent of the model provided here. It has its own
41dependency on a target kernel release where converted code is built
42and executed. Any change in kernel APIs essential to klitmus7 will
43necessitate an upgrade of klitmus7.
44
45If you find any compatibility issues in klitmus7, please inform the
46memory model maintainers.
47
48klitmus7 Compatibility Table
49----------------------------
50
51 ============ ==========
52 target Linux herdtools7
53 ------------ ----------
54 -- 4.14 7.48 --
55 4.15 -- 4.19 7.49 --
56 4.20 -- 5.5 7.54 --
57 5.6 -- 5.16 7.56 --
58 5.17 -- 7.56.1 --
59 ============ ==========
60
61
62==================
63BASIC USAGE: HERD7
64==================
65
66The memory model is used, in conjunction with "herd7", to exhaustively
67explore the state space of small litmus tests. Documentation describing
68the format, features, capabilities and limitations of these litmus
69tests is available in tools/memory-model/Documentation/litmus-tests.txt.
70
71Example litmus tests may be found in the Linux-kernel source tree:
72
73 tools/memory-model/litmus-tests/
74 Documentation/litmus-tests/
75
76Several thousand more example litmus tests are available here:
77
78 https://github.com/paulmckrcu/litmus
79 https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/herd
80 https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/litmus
81
82Documentation describing litmus tests and now to use them may be found
83here:
84
85 tools/memory-model/Documentation/litmus-tests.txt
86
87The remainder of this section uses the SB+fencembonceonces.litmus test
88located in the tools/memory-model directory.
89
90To run SB+fencembonceonces.litmus against the memory model:
91
92 $ cd $LINUX_SOURCE_TREE/tools/memory-model
93 $ herd7 -conf linux-kernel.cfg litmus-tests/SB+fencembonceonces.litmus
94
95Here is the corresponding output:
96
97 Test SB+fencembonceonces Allowed
98 States 3
99 0:r0=0; 1:r0=1;
100 0:r0=1; 1:r0=0;
101 0:r0=1; 1:r0=1;
102 No
103 Witnesses
104 Positive: 0 Negative: 3
105 Condition exists (0:r0=0 /\ 1:r0=0)
106 Observation SB+fencembonceonces Never 0 3
107 Time SB+fencembonceonces 0.01
108 Hash=d66d99523e2cac6b06e66f4c995ebb48
109
110The "Positive: 0 Negative: 3" and the "Never 0 3" each indicate that
111this litmus test's "exists" clause can not be satisfied.
112
113See "herd7 -help" or "herdtools7/doc/" for more information on running the
114tool itself, but please be aware that this documentation is intended for
115people who work on the memory model itself, that is, people making changes
116to the tools/memory-model/linux-kernel.* files. It is not intended for
117people focusing on writing, understanding, and running LKMM litmus tests.
118
119
120=====================
121BASIC USAGE: KLITMUS7
122=====================
123
124The "klitmus7" tool converts a litmus test into a Linux kernel module,
125which may then be loaded and run.
126
127For example, to run SB+fencembonceonces.litmus against hardware:
128
129 $ mkdir mymodules
130 $ klitmus7 -o mymodules litmus-tests/SB+fencembonceonces.litmus
131 $ cd mymodules ; make
132 $ sudo sh run.sh
133
134The corresponding output includes:
135
136 Test SB+fencembonceonces Allowed
137 Histogram (3 states)
138 644580 :>0:r0=1; 1:r0=0;
139 644328 :>0:r0=0; 1:r0=1;
140 711092 :>0:r0=1; 1:r0=1;
141 No
142 Witnesses
143 Positive: 0, Negative: 2000000
144 Condition exists (0:r0=0 /\ 1:r0=0) is NOT validated
145 Hash=d66d99523e2cac6b06e66f4c995ebb48
146 Observation SB+fencembonceonces Never 0 2000000
147 Time SB+fencembonceonces 0.16
148
149The "Positive: 0 Negative: 2000000" and the "Never 0 2000000" indicate
150that during two million trials, the state specified in this litmus
151test's "exists" clause was not reached.
152
153And, as with "herd7", please see "klitmus7 -help" or "herdtools7/doc/"
154for more information. And again, please be aware that this documentation
155is intended for people who work on the memory model itself, that is,
156people making changes to the tools/memory-model/linux-kernel.* files.
157It is not intended for people focusing on writing, understanding, and
158running LKMM litmus tests.
159
160
161====================
162DESCRIPTION OF FILES
163====================
164
165Documentation/README
166 Guide to the other documents in the Documentation/ directory.
167
168linux-kernel.bell
169 Categorizes the relevant instructions, including memory
170 references, memory barriers, atomic read-modify-write operations,
171 lock acquisition/release, and RCU operations.
172
173 More formally, this file (1) lists the subtypes of the various
174 event types used by the memory model and (2) performs RCU
175 read-side critical section nesting analysis.
176
177linux-kernel.cat
178 Specifies what reorderings are forbidden by memory references,
179 memory barriers, atomic read-modify-write operations, and RCU.
180
181 More formally, this file specifies what executions are forbidden
182 by the memory model. Allowed executions are those which
183 satisfy the model's "coherence", "atomic", "happens-before",
184 "propagation", and "rcu" axioms, which are defined in the file.
185
186linux-kernel.cfg
187 Convenience file that gathers the common-case herd7 command-line
188 arguments.
189
190linux-kernel.def
191 Maps from C-like syntax to herd7's internal litmus-test
192 instruction-set architecture.
193
194litmus-tests
195 Directory containing a few representative litmus tests, which
196 are listed in litmus-tests/README. A great deal more litmus
197 tests are available at https://github.com/paulmckrcu/litmus.
198
199 By "representative", it means the one in the litmus-tests
200 directory is:
201
202 1) simple, the number of threads should be relatively
203 small and each thread function should be relatively
204 simple.
205 2) orthogonal, there should be no two litmus tests
206 describing the same aspect of the memory model.
207 3) textbook, developers can easily copy-paste-modify
208 the litmus tests to use the patterns on their own
209 code.
210
211lock.cat
212 Provides a front-end analysis of lock acquisition and release,
213 for example, associating a lock acquisition with the preceding
214 and following releases and checking for self-deadlock.
215
216 More formally, this file defines a performance-enhanced scheme
217 for generation of the possible reads-from and coherence order
218 relations on the locking primitives.
219
220README
221 This file.
222
223scripts Various scripts, see scripts/README.
224