The C11 and C++11 Concurrency Model

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320 The thread-local semantics collects these syntactic dependencies in the data dependence relation, dd, and that is passed to the memory model as part of the pre-execution. The relation is a partial order that identifies a subset of sequenced-before where there is data dependence between memory actions. The restrictions imposed on the relation in well formed threads are: strict partial order Xo.actions Xo.dd ∧ Xo.dd subset Xo.sb B.1.6 Well-formed threads The action set, location kinds and relations of the pre-execution are generated by the thread-local semantics. The pre-executions that the thread-local semantics generates should all satisfy the basic properties established above. The well formed threads predicate gathers those requirements together: let well formed threads ((Xo, , ) : (pre execution ∗ execution witness ∗ relation list)) = (∀ a ∈ Xo.actions. well formed action a) ∧ actions respect location kinds Xo.actions Xo.lk ∧ blocking observed Xo.actions Xo.sb ∧ inj on aid of Xo.actions ∧ relation over Xo.actions Xo.sb ∧ relation over Xo.actions Xo.asw ∧ threadwise Xo.actions Xo.sb ∧ interthread Xo.actions Xo.asw ∧ isStrictPartialOrder Xo.sb ∧ isStrictPartialOrder Xo.dd ∧ Xo.dd ⊆ Xo.sb ∧ indeterminate sequencing Xo ∧ isIrreflexive (sbasw Xo) ∧ finite prefixes (sbasw Xo) Xo.actions B.2 Execution witness The execution witness represents the dynamic behaviour of memory in an execution. It is defined in the model as a record containing several relations: type execution witness = 〈| rf : set (action ∗ action); mo : set (action ∗ action);
321 |〉 sc : set (action ∗ action); lo : set (action ∗ action); tot : set (action ∗ action); Each model given in Chapter 3 used some subset of the relations. The standard model used rf, mo, lo and sc, and each of these are justified by the standard below. B.2.1 Reads-from The behaviour of reads from memory is described in 1.10: 1.10p3 The value of an object visible to a thread T at a particular point is the initial value of the object, a value assigned to the object by T, or a value assigned to the object by another thread, according to the rules below. [...] Theexcerptrequiresareadtoreaditsvaluefromaparticularwrite. Consequently, the model records the particular write that a read reads from, and collects all such relationships together in the reads-from relation. The model checks some basic sanity properties over the relation: that it is an injective relation from write actions to read actions at the same location, and that each read returns the same value as the related write: let well formed rf (Xo, Xw, ) = ∀ (a, b) ∈ Xw.rf. a ∈ Xo.actions ∧ b ∈ Xo.actions ∧ loc of a = loc of b ∧ is write a ∧ is read b ∧ value read by b = value written by a ∧ ∀ a ′ ∈ Xo.actions. (a ′ , b) ∈ Xw.rf −→ a = a ′ In the same passage, there is a note that alludes to the possibility of undefined behaviour that will be discussed in Section B.5.3: 1.10p3 [...][ Note: In some cases, there may instead be undefined behavior. Much of this section is motivated by the desire to support atomic operations with explicit and detailed visibility constraints. However, it also implicitly supports a simpler view for more restricted programs. — end note ]
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The C11 and C++11 Concurrency Model
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Mark John Batty The C11 and C++11 C
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8 3.5.1 Release sequences . . . . .
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10 B.1 The pre-execution type . . .
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12
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14 stores to memory are interleaved
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16 still, relaxed-concurrencybugsca
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18 to sequential consistency [37],
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20 ory model. It has been used for
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22 by Dubois et al. [48]. C/C++11 c
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24 There has been some work on veri
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26 int x = 0; int y = 0; x = 1; y =
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28 On the SC memory model this prog
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30 The x86 architecture provides th
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32 Write request Read request Barri
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34 coherence-commitment order restr
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36 Multi-copy atomicity Some memory
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38 Further details of the Power and
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40 they can be guaranteed with no d
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42 indivisible events that affect t
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44 undefined behaviour. In the prog
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46 On each architecture, this is su
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48
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50 sublanguages and the related par
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52 int main() { int x = 2; int y =
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54 a:W NA x=0 sb int main() { int x
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56 effects of a particular read are
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58 | Blocked rmw l → lk l = Atomi
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60 let det read (Xo, Xw, :: (“vse
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62 let indeterminate reads (Xo, Xw,
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64 let single thread memory model =
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66 becomes: int main() { int x = 0;
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68 additional-synchronises-withedge
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70 let locks only consistent locks
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72 let data races (Xo, Xw, (“hb
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74 = ( (¬ (a = b)) ∧ is write a
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76 the atomic location is accessed.
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78 Message passing, MP The first ex
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80 a:W NA x=0 sb rf b:W NA y=0 rf r
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82 the C/C++11 analogue of the test
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84 On Power and ARM, the analogue o
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86 stores of this fragment of the l
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88 int main() { atomic_int x = 0; a
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90 int main() { int x = 0; atomic_i
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92 and ARM all guarantee that each
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94 Release fences In the example in
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96 ( is fence a ∧ is release a
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98 3.7 Programs with SC atomics Pro
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100 not forbid the store-buffering
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102 int main() { atomic_int x = 0;
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104 ( (w, w ′ ) ∈ Xw.mo ∧ (w
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108 conjunct of sc fenced sc fences
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110 int main() { int x = 0; atomic_
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112 let r = sw ∪ dob ∪ (compose
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114 of the read. This write forms t
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116
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118 behaviour of the memory model.
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120 At the top right, there are con
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124 A release sequence headed by a
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126 [ Note: The visible sequence of
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130 5.6 Undefined loops In C/C++11
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132 Next consider the execution bel
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134 ultimately faulty specification
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136 air problem. It seems clear the
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138 void main() { atomic_int x = 0;
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140 5.10.2 Possible solutions One c
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142 programs with thin-air values w
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144 Java introduces a great deal of
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148
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150 standard model T. 1 with consum
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152 tions, or they both produce und
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154 the two models are almost ident
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156 h, in the acyclic relation mo.
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158 | RMW mo → (mo ∈ {Acq rel,
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160 Programs without SC atomics Rem
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162 match a with | Lock → true |
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164 Theorem 8. (∀ opsem p. static
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166 is release rel ∧ ( (b = rel)
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168 Now we show that this equivalen
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170 is a dynamic property that can
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172 standard model T. 1 with consum
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174 [...][ Note: It can be shown th
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176 • consistent hb Furthermore,
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178 | Load mo → (mo ∈ {NA, Seq
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180 The induction will proceed by a
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182 exists a consistent execution i
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184 The domain and range of hbscr i
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186 implies that there is no tot in
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188 There are two directions to est
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190 incorporates the new action. Th
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192 Now for each sort of fault, we
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194 Theorem 14. For a program that
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196 we need only show that the sc-o
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198
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200 This chapter presents theorems
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202 C++0x actions a:W NA x=1 d:R AC
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204 and consume atomics require the
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206 Thread 0 has an lwsync as in th
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208 The thread and storage subsyste
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210 we know that any inter-thread h
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212
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214 behaviour in the specification.
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216 atomic Seq S; void init() { sto
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218 Message passing (MP): int a, b,
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220 in composition with a client wi
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222 tation in an arbitrary client c
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224 push and pop in an execution. T
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226 execution of the component exte
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228 Then for some Z ∈ C(L 2 )(I
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230
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232 it is possible to identify erra
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234 The release-sequence of C/C++11
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236 mer’s memory model. The CPU a
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238 17.3 defines additional terms t
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240 and the other is not, or if the
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242 abstract machine with the same
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244 a = ((a + b) + 32765); since if
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246 | RMW of aid ∗ tid ∗ memory
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248 not otherwise specifically sequ
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250 gram can potentially access eve
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252 acquire fence, a release fence,
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254 This relation does not order th
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256 performs a consume operation on
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258 • A is sequenced before B, or
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260 The model represents visible si
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262 where the three relations disag
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264 CoRW In this coherence violatio
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266 referred to as “sequential co
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268 29 Atomic operations library [a
Page 270 and 271: 270 In the model, the memory order
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Page 274 and 275: 274 4 For an atomic operation B tha
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Page 282 and 283: 282 The model captures these potent
Page 284 and 285: 284 bool atomic_compare_exchange_we
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Page 288 and 289: 288 31 Effects: fetch key(operand)
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Page 292 and 293: 292 extern "C" void atomic_signal_f
Page 294 and 295: 294 30.3.1.2 thread constructors [t
Page 296 and 297: 296 Header synopsis [elided] 30.4.
Page 298 and 299: 298 The behaviour of locks and unlo
Page 300 and 301: 300 9 Postcondition: The calling th
Page 302 and 303: 302 let locks only bad mutexes (Xo,
Page 304 and 305: 304 after the unlock call returns.
Page 306 and 307: 306 |〉 threads : set (tid); lk :
Page 308 and 309: 308 let aid of a = match a with | L
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Page 312 and 313: 312 atomic location. On failure the
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Page 316 and 317: 316 | Fence mo → mo ∈ {Release,
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Page 346 and 347: 346 To cover the second and third c
Page 348 and 349: 348 This restriction requires reads
Page 350 and 351: 350 [...]If a side effect on a scal
Page 352 and 353: 352 The model captures these potent
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Page 356 and 357: 356 each empty M.undefined X then D
Page 358 and 359: 358 type program impl = nat type ti
Page 360 and 361: 360 let named predicate tree measur
Page 362 and 363: 362 C.3 Projection functions let ai
Page 364 and 365: 364 | RMW → true | → false end
Page 366 and 367: 366 | Store mo → mo = Seq cst | R
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370 a ′ ∈ fringe set Xo ′ A
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372 val indeterminate reads : candi
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374 let locks only consistent locks
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376 val locks only behaviour : ∀
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378 |〉〈| rf flag = true; mo fl
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380 (“release acquire coherent me
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382 val release acquire relaxed beh
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384 behaviourrelease acquire fenced
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386 ∀ (Xo, Xw, rl) ∈ Xs. ∀ a
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388 let sc fenced behaviour opsem (
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390 behaviour with consume memory m
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392 let release acquire SC conditio
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394 let bounded executions (Xs : se
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396 { (a, b) | ∀ a ∈ Xo.actions
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398 statically satisfied single thr
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400 let vse = visible side effect s
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402
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404 [15] J. Alglave, D. Kroening, V
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406 [42] L. Censier and P. Feautrie
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408 [68] S. Mador-Haim, L. Maranget
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410 ization and analysis of message
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Index actions, 41, 245 additional s
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414 Last updated: Saturday 29 th No
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The C11 and C++11 Concurrency Model

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