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. Therefore, if a later version does 32not work, please try using the exact version called out above. 33 34 35================== 36BASIC USAGE: HERD7 37================== 38 39The memory model is used, in conjunction with "herd7", to exhaustively 40explore the state space of small litmus tests. 41 42For example, to run SB+fencembonceonces.litmus against the memory model: 43 44 $ herd7 -conf linux-kernel.cfg litmus-tests/SB+fencembonceonces.litmus 45 46Here is the corresponding output: 47 48 Test SB+fencembonceonces Allowed 49 States 3 50 0:r0=0; 1:r0=1; 51 0:r0=1; 1:r0=0; 52 0:r0=1; 1:r0=1; 53 No 54 Witnesses 55 Positive: 0 Negative: 3 56 Condition exists (0:r0=0 /\ 1:r0=0) 57 Observation SB+fencembonceonces Never 0 3 58 Time SB+fencembonceonces 0.01 59 Hash=d66d99523e2cac6b06e66f4c995ebb48 60 61The "Positive: 0 Negative: 3" and the "Never 0 3" each indicate that 62this litmus test's "exists" clause can not be satisfied. 63 64See "herd7 -help" or "herdtools7/doc/" for more information. 65 66 67===================== 68BASIC USAGE: KLITMUS7 69===================== 70 71The "klitmus7" tool converts a litmus test into a Linux kernel module, 72which may then be loaded and run. 73 74For example, to run SB+fencembonceonces.litmus against hardware: 75 76 $ mkdir mymodules 77 $ klitmus7 -o mymodules litmus-tests/SB+fencembonceonces.litmus 78 $ cd mymodules ; make 79 $ sudo sh run.sh 80 81The corresponding output includes: 82 83 Test SB+fencembonceonces Allowed 84 Histogram (3 states) 85 644580 :>0:r0=1; 1:r0=0; 86 644328 :>0:r0=0; 1:r0=1; 87 711092 :>0:r0=1; 1:r0=1; 88 No 89 Witnesses 90 Positive: 0, Negative: 2000000 91 Condition exists (0:r0=0 /\ 1:r0=0) is NOT validated 92 Hash=d66d99523e2cac6b06e66f4c995ebb48 93 Observation SB+fencembonceonces Never 0 2000000 94 Time SB+fencembonceonces 0.16 95 96The "Positive: 0 Negative: 2000000" and the "Never 0 2000000" indicate 97that during two million trials, the state specified in this litmus 98test's "exists" clause was not reached. 99 100And, as with "herd7", please see "klitmus7 -help" or "herdtools7/doc/" 101for more information. 102 103 104==================== 105DESCRIPTION OF FILES 106==================== 107 108Documentation/cheatsheet.txt 109 Quick-reference guide to the Linux-kernel memory model. 110 111Documentation/explanation.txt 112 Describes the memory model in detail. 113 114Documentation/recipes.txt 115 Lists common memory-ordering patterns. 116 117Documentation/references.txt 118 Provides background reading. 119 120linux-kernel.bell 121 Categorizes the relevant instructions, including memory 122 references, memory barriers, atomic read-modify-write operations, 123 lock acquisition/release, and RCU operations. 124 125 More formally, this file (1) lists the subtypes of the various 126 event types used by the memory model and (2) performs RCU 127 read-side critical section nesting analysis. 128 129linux-kernel.cat 130 Specifies what reorderings are forbidden by memory references, 131 memory barriers, atomic read-modify-write operations, and RCU. 132 133 More formally, this file specifies what executions are forbidden 134 by the memory model. Allowed executions are those which 135 satisfy the model's "coherence", "atomic", "happens-before", 136 "propagation", and "rcu" axioms, which are defined in the file. 137 138linux-kernel.cfg 139 Convenience file that gathers the common-case herd7 command-line 140 arguments. 141 142linux-kernel.def 143 Maps from C-like syntax to herd7's internal litmus-test 144 instruction-set architecture. 145 146litmus-tests 147 Directory containing a few representative litmus tests, which 148 are listed in litmus-tests/README. A great deal more litmus 149 tests are available at https://github.com/paulmckrcu/litmus. 150 151lock.cat 152 Provides a front-end analysis of lock acquisition and release, 153 for example, associating a lock acquisition with the preceding 154 and following releases and checking for self-deadlock. 155 156 More formally, this file defines a performance-enhanced scheme 157 for generation of the possible reads-from and coherence order 158 relations on the locking primitives. 159 160README 161 This file. 162 163scripts Various scripts, see scripts/README. 164 165 166=========== 167LIMITATIONS 168=========== 169 170The Linux-kernel memory model has the following limitations: 171 1721. Compiler optimizations are not modeled. Of course, the use 173 of READ_ONCE() and WRITE_ONCE() limits the compiler's ability 174 to optimize, but there is Linux-kernel code that uses bare C 175 memory accesses. Handling this code is on the to-do list. 176 For more information, see Documentation/explanation.txt (in 177 particular, the "THE PROGRAM ORDER RELATION: po AND po-loc" 178 and "A WARNING" sections). 179 180 Note that this limitation in turn limits LKMM's ability to 181 accurately model address, control, and data dependencies. 182 For example, if the compiler can deduce the value of some variable 183 carrying a dependency, then the compiler can break that dependency 184 by substituting a constant of that value. 185 1862. Multiple access sizes for a single variable are not supported, 187 and neither are misaligned or partially overlapping accesses. 188 1893. Exceptions and interrupts are not modeled. In some cases, 190 this limitation can be overcome by modeling the interrupt or 191 exception with an additional process. 192 1934. I/O such as MMIO or DMA is not supported. 194 1955. Self-modifying code (such as that found in the kernel's 196 alternatives mechanism, function tracer, Berkeley Packet Filter 197 JIT compiler, and module loader) is not supported. 198 1996. Complete modeling of all variants of atomic read-modify-write 200 operations, locking primitives, and RCU is not provided. 201 For example, call_rcu() and rcu_barrier() are not supported. 202 However, a substantial amount of support is provided for these 203 operations, as shown in the linux-kernel.def file. 204 205 a. When rcu_assign_pointer() is passed NULL, the Linux 206 kernel provides no ordering, but LKMM models this 207 case as a store release. 208 209 b. The "unless" RMW operations are not currently modeled: 210 atomic_long_add_unless(), atomic_add_unless(), 211 atomic_inc_unless_negative(), and 212 atomic_dec_unless_positive(). These can be emulated 213 in litmus tests, for example, by using atomic_cmpxchg(). 214 215 c. The call_rcu() function is not modeled. It can be 216 emulated in litmus tests by adding another process that 217 invokes synchronize_rcu() and the body of the callback 218 function, with (for example) a release-acquire from 219 the site of the emulated call_rcu() to the beginning 220 of the additional process. 221 222 d. The rcu_barrier() function is not modeled. It can be 223 emulated in litmus tests emulating call_rcu() via 224 (for example) a release-acquire from the end of each 225 additional call_rcu() process to the site of the 226 emulated rcu-barrier(). 227 228 e. Although sleepable RCU (SRCU) is now modeled, there 229 are some subtle differences between its semantics and 230 those in the Linux kernel. For example, the kernel 231 might interpret the following sequence as two partially 232 overlapping SRCU read-side critical sections: 233 234 1 r1 = srcu_read_lock(&my_srcu); 235 2 do_something_1(); 236 3 r2 = srcu_read_lock(&my_srcu); 237 4 do_something_2(); 238 5 srcu_read_unlock(&my_srcu, r1); 239 6 do_something_3(); 240 7 srcu_read_unlock(&my_srcu, r2); 241 242 In contrast, LKMM will interpret this as a nested pair of 243 SRCU read-side critical sections, with the outer critical 244 section spanning lines 1-7 and the inner critical section 245 spanning lines 3-5. 246 247 This difference would be more of a concern had anyone 248 identified a reasonable use case for partially overlapping 249 SRCU read-side critical sections. For more information, 250 please see: https://paulmck.livejournal.com/40593.html 251 252 f. Reader-writer locking is not modeled. It can be 253 emulated in litmus tests using atomic read-modify-write 254 operations. 255 256The "herd7" tool has some additional limitations of its own, apart from 257the memory model: 258 2591. Non-trivial data structures such as arrays or structures are 260 not supported. However, pointers are supported, allowing trivial 261 linked lists to be constructed. 262 2632. Dynamic memory allocation is not supported, although this can 264 be worked around in some cases by supplying multiple statically 265 allocated variables. 266 267Some of these limitations may be overcome in the future, but others are 268more likely to be addressed by incorporating the Linux-kernel memory model 269into other tools. 270 271Finally, please note that LKMM is subject to change as hardware, use cases, 272and compilers evolve. 273