1 ============================== 1 =====================================
2 LINUX KERNEL MEMORY CONSISTENC 2 LINUX KERNEL MEMORY CONSISTENCY MODEL
3 ============================== 3 =====================================
4 4
5 ============ 5 ============
6 INTRODUCTION 6 INTRODUCTION
7 ============ 7 ============
8 8
9 This directory contains the memory consistency 9 This directory contains the memory consistency model (memory model, for
10 short) of the Linux kernel, written in the "ca 10 short) of the Linux kernel, written in the "cat" language and executable
11 by the externally provided "herd7" simulator, 11 by the externally provided "herd7" simulator, which exhaustively explores
12 the state space of small litmus tests. 12 the state space of small litmus tests.
13 13
14 In addition, the "klitmus7" tool (also externa 14 In addition, the "klitmus7" tool (also externally provided) may be used
15 to convert a litmus test to a Linux kernel mod 15 to convert a litmus test to a Linux kernel module, which in turn allows
16 that litmus test to be exercised within the Li 16 that litmus test to be exercised within the Linux kernel.
17 17
18 18
19 ============ 19 ============
20 REQUIREMENTS 20 REQUIREMENTS
21 ============ 21 ============
22 22
23 Version 7.52 or higher of the "herd7" and "kli 23 Version 7.52 or higher of the "herd7" and "klitmus7" tools must be
24 downloaded separately: 24 downloaded separately:
25 25
26 https://github.com/herd/herdtools7 26 https://github.com/herd/herdtools7
27 27
28 See "herdtools7/INSTALL.md" for installation i 28 See "herdtools7/INSTALL.md" for installation instructions.
29 29
30 Note that although these tools usually provide 30 Note that although these tools usually provide backwards compatibility,
31 this is not absolutely guaranteed. 31 this is not absolutely guaranteed.
32 32
33 For example, a future version of herd7 might n 33 For example, a future version of herd7 might not work with the model
34 in this release. A compatible model will like 34 in this release. A compatible model will likely be made available in
35 a later release of Linux kernel. 35 a later release of Linux kernel.
36 36
37 If you absolutely need to run the model in thi 37 If you absolutely need to run the model in this particular release,
38 please try using the exact version called out 38 please try using the exact version called out above.
39 39
40 klitmus7 is independent of the model provided 40 klitmus7 is independent of the model provided here. It has its own
41 dependency on a target kernel release where co 41 dependency on a target kernel release where converted code is built
42 and executed. Any change in kernel APIs essen 42 and executed. Any change in kernel APIs essential to klitmus7 will
43 necessitate an upgrade of klitmus7. 43 necessitate an upgrade of klitmus7.
44 44
45 If you find any compatibility issues in klitmu 45 If you find any compatibility issues in klitmus7, please inform the
46 memory model maintainers. 46 memory model maintainers.
47 47
48 klitmus7 Compatibility Table 48 klitmus7 Compatibility Table
49 ---------------------------- 49 ----------------------------
50 50
51 ============ ========== 51 ============ ==========
52 target Linux herdtools7 52 target Linux herdtools7
53 ------------ ---------- 53 ------------ ----------
54 -- 4.14 7.48 -- 54 -- 4.14 7.48 --
55 4.15 -- 4.19 7.49 -- 55 4.15 -- 4.19 7.49 --
56 4.20 -- 5.5 7.54 -- 56 4.20 -- 5.5 7.54 --
57 5.6 -- 5.16 7.56 -- 57 5.6 -- 5.16 7.56 --
58 5.17 -- 7.56.1 -- 58 5.17 -- 7.56.1 --
59 ============ ========== 59 ============ ==========
60 60
61 61
62 ================== 62 ==================
63 BASIC USAGE: HERD7 63 BASIC USAGE: HERD7
64 ================== 64 ==================
65 65
66 The memory model is used, in conjunction with 66 The memory model is used, in conjunction with "herd7", to exhaustively
67 explore the state space of small litmus tests. 67 explore the state space of small litmus tests. Documentation describing
68 the format, features, capabilities and limitat 68 the format, features, capabilities and limitations of these litmus
69 tests is available in tools/memory-model/Docum 69 tests is available in tools/memory-model/Documentation/litmus-tests.txt.
70 70
71 Example litmus tests may be found in the Linux 71 Example litmus tests may be found in the Linux-kernel source tree:
72 72
73 tools/memory-model/litmus-tests/ 73 tools/memory-model/litmus-tests/
74 Documentation/litmus-tests/ 74 Documentation/litmus-tests/
75 75
76 Several thousand more example litmus tests are 76 Several thousand more example litmus tests are available here:
77 77
78 https://github.com/paulmckrcu/litmus 78 https://github.com/paulmckrcu/litmus
79 https://git.kernel.org/pub/scm/linux/k 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/k 80 https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/litmus
81 81
82 Documentation describing litmus tests and now 82 Documentation describing litmus tests and now to use them may be found
83 here: 83 here:
84 84
85 tools/memory-model/Documentation/litmu 85 tools/memory-model/Documentation/litmus-tests.txt
86 86
87 The remainder of this section uses the SB+fenc 87 The remainder of this section uses the SB+fencembonceonces.litmus test
88 located in the tools/memory-model directory. 88 located in the tools/memory-model directory.
89 89
90 To run SB+fencembonceonces.litmus against the 90 To run SB+fencembonceonces.litmus against the memory model:
91 91
92 $ cd $LINUX_SOURCE_TREE/tools/memory-model 92 $ cd $LINUX_SOURCE_TREE/tools/memory-model
93 $ herd7 -conf linux-kernel.cfg litmus-tests/ 93 $ herd7 -conf linux-kernel.cfg litmus-tests/SB+fencembonceonces.litmus
94 94
95 Here is the corresponding output: 95 Here is the corresponding output:
96 96
97 Test SB+fencembonceonces Allowed 97 Test SB+fencembonceonces Allowed
98 States 3 98 States 3
99 0:r0=0; 1:r0=1; 99 0:r0=0; 1:r0=1;
100 0:r0=1; 1:r0=0; 100 0:r0=1; 1:r0=0;
101 0:r0=1; 1:r0=1; 101 0:r0=1; 1:r0=1;
102 No 102 No
103 Witnesses 103 Witnesses
104 Positive: 0 Negative: 3 104 Positive: 0 Negative: 3
105 Condition exists (0:r0=0 /\ 1:r0=0) 105 Condition exists (0:r0=0 /\ 1:r0=0)
106 Observation SB+fencembonceonces Never 0 3 106 Observation SB+fencembonceonces Never 0 3
107 Time SB+fencembonceonces 0.01 107 Time SB+fencembonceonces 0.01
108 Hash=d66d99523e2cac6b06e66f4c995ebb48 108 Hash=d66d99523e2cac6b06e66f4c995ebb48
109 109
110 The "Positive: 0 Negative: 3" and the "Never 0 110 The "Positive: 0 Negative: 3" and the "Never 0 3" each indicate that
111 this litmus test's "exists" clause can not be 111 this litmus test's "exists" clause can not be satisfied.
112 112
113 See "herd7 -help" or "herdtools7/doc/" for mor 113 See "herd7 -help" or "herdtools7/doc/" for more information on running the
114 tool itself, but please be aware that this doc 114 tool itself, but please be aware that this documentation is intended for
115 people who work on the memory model itself, th 115 people who work on the memory model itself, that is, people making changes
116 to the tools/memory-model/linux-kernel.* files 116 to the tools/memory-model/linux-kernel.* files. It is not intended for
117 people focusing on writing, understanding, and 117 people focusing on writing, understanding, and running LKMM litmus tests.
118 118
119 119
120 ===================== 120 =====================
121 BASIC USAGE: KLITMUS7 121 BASIC USAGE: KLITMUS7
122 ===================== 122 =====================
123 123
124 The "klitmus7" tool converts a litmus test int 124 The "klitmus7" tool converts a litmus test into a Linux kernel module,
125 which may then be loaded and run. 125 which may then be loaded and run.
126 126
127 For example, to run SB+fencembonceonces.litmus 127 For example, to run SB+fencembonceonces.litmus against hardware:
128 128
129 $ mkdir mymodules 129 $ mkdir mymodules
130 $ klitmus7 -o mymodules litmus-tests/SB+fenc 130 $ klitmus7 -o mymodules litmus-tests/SB+fencembonceonces.litmus
131 $ cd mymodules ; make 131 $ cd mymodules ; make
132 $ sudo sh run.sh 132 $ sudo sh run.sh
133 133
134 The corresponding output includes: 134 The corresponding output includes:
135 135
136 Test SB+fencembonceonces Allowed 136 Test SB+fencembonceonces Allowed
137 Histogram (3 states) 137 Histogram (3 states)
138 644580 :>0:r0=1; 1:r0=0; 138 644580 :>0:r0=1; 1:r0=0;
139 644328 :>0:r0=0; 1:r0=1; 139 644328 :>0:r0=0; 1:r0=1;
140 711092 :>0:r0=1; 1:r0=1; 140 711092 :>0:r0=1; 1:r0=1;
141 No 141 No
142 Witnesses 142 Witnesses
143 Positive: 0, Negative: 2000000 143 Positive: 0, Negative: 2000000
144 Condition exists (0:r0=0 /\ 1:r0=0) is NOT v 144 Condition exists (0:r0=0 /\ 1:r0=0) is NOT validated
145 Hash=d66d99523e2cac6b06e66f4c995ebb48 145 Hash=d66d99523e2cac6b06e66f4c995ebb48
146 Observation SB+fencembonceonces Never 0 2000 146 Observation SB+fencembonceonces Never 0 2000000
147 Time SB+fencembonceonces 0.16 147 Time SB+fencembonceonces 0.16
148 148
149 The "Positive: 0 Negative: 2000000" and the "N 149 The "Positive: 0 Negative: 2000000" and the "Never 0 2000000" indicate
150 that during two million trials, the state spec 150 that during two million trials, the state specified in this litmus
151 test's "exists" clause was not reached. 151 test's "exists" clause was not reached.
152 152
153 And, as with "herd7", please see "klitmus7 -he 153 And, as with "herd7", please see "klitmus7 -help" or "herdtools7/doc/"
154 for more information. And again, please be aw 154 for more information. And again, please be aware that this documentation
155 is intended for people who work on the memory 155 is intended for people who work on the memory model itself, that is,
156 people making changes to the tools/memory-mode 156 people making changes to the tools/memory-model/linux-kernel.* files.
157 It is not intended for people focusing on writ 157 It is not intended for people focusing on writing, understanding, and
158 running LKMM litmus tests. 158 running LKMM litmus tests.
159 159
160 160
161 ==================== 161 ====================
162 DESCRIPTION OF FILES 162 DESCRIPTION OF FILES
163 ==================== 163 ====================
164 164
165 Documentation/README 165 Documentation/README
166 Guide to the other documents in the Do 166 Guide to the other documents in the Documentation/ directory.
167 167
168 linux-kernel.bell 168 linux-kernel.bell
169 Categorizes the relevant instructions, 169 Categorizes the relevant instructions, including memory
170 references, memory barriers, atomic re 170 references, memory barriers, atomic read-modify-write operations,
171 lock acquisition/release, and RCU oper 171 lock acquisition/release, and RCU operations.
172 172
173 More formally, this file (1) lists the 173 More formally, this file (1) lists the subtypes of the various
174 event types used by the memory model a 174 event types used by the memory model and (2) performs RCU
175 read-side critical section nesting ana 175 read-side critical section nesting analysis.
176 176
177 linux-kernel.cat 177 linux-kernel.cat
178 Specifies what reorderings are forbidd 178 Specifies what reorderings are forbidden by memory references,
179 memory barriers, atomic read-modify-wr 179 memory barriers, atomic read-modify-write operations, and RCU.
180 180
181 More formally, this file specifies wha 181 More formally, this file specifies what executions are forbidden
182 by the memory model. Allowed executio 182 by the memory model. Allowed executions are those which
183 satisfy the model's "coherence", "atom 183 satisfy the model's "coherence", "atomic", "happens-before",
184 "propagation", and "rcu" axioms, which 184 "propagation", and "rcu" axioms, which are defined in the file.
185 185
186 linux-kernel.cfg 186 linux-kernel.cfg
187 Convenience file that gathers the comm 187 Convenience file that gathers the common-case herd7 command-line
188 arguments. 188 arguments.
189 189
190 linux-kernel.def 190 linux-kernel.def
191 Maps from C-like syntax to herd7's int 191 Maps from C-like syntax to herd7's internal litmus-test
192 instruction-set architecture. 192 instruction-set architecture.
193 193
194 litmus-tests 194 litmus-tests
195 Directory containing a few representat 195 Directory containing a few representative litmus tests, which
196 are listed in litmus-tests/README. A 196 are listed in litmus-tests/README. A great deal more litmus
197 tests are available at https://github. 197 tests are available at https://github.com/paulmckrcu/litmus.
198 198
199 By "representative", it means the one 199 By "representative", it means the one in the litmus-tests
200 directory is: 200 directory is:
201 201
202 1) simple, the number of threa 202 1) simple, the number of threads should be relatively
203 small and each thread funct 203 small and each thread function should be relatively
204 simple. 204 simple.
205 2) orthogonal, there should be 205 2) orthogonal, there should be no two litmus tests
206 describing the same aspect 206 describing the same aspect of the memory model.
207 3) textbook, developers can ea 207 3) textbook, developers can easily copy-paste-modify
208 the litmus tests to use the 208 the litmus tests to use the patterns on their own
209 code. 209 code.
210 210
211 lock.cat 211 lock.cat
212 Provides a front-end analysis of lock 212 Provides a front-end analysis of lock acquisition and release,
213 for example, associating a lock acquis 213 for example, associating a lock acquisition with the preceding
214 and following releases and checking fo 214 and following releases and checking for self-deadlock.
215 215
216 More formally, this file defines a per 216 More formally, this file defines a performance-enhanced scheme
217 for generation of the possible reads-f 217 for generation of the possible reads-from and coherence order
218 relations on the locking primitives. 218 relations on the locking primitives.
219 219
220 README 220 README
221 This file. 221 This file.
222 222
223 scripts Various scripts, see scripts/README. 223 scripts Various scripts, see scripts/README.
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