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examples/classic/distributed/TeachingConcurrency/teachingConcurrency.qnt
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// -*- mode: Bluespec; -*- | ||
/** | ||
* A simple algorithm from Lamport's "Teaching Concurrency" paper: | ||
* https://lamport.azurewebsites.net/pubs/teaching-concurrency.pdf | ||
* | ||
* Run in verifier with: | ||
* quint verify --temporal correct teachingConcurrency.qnt | ||
*/ | ||
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// An example instance of the algorithm with N=5. | ||
module teachingConcurrency { | ||
import teachingConcurrencyN(NUM_PROCS=5).* | ||
} | ||
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// A concurrency algorithm providing a basic form of mutual | ||
// exclusion. For details and proofs see section 7.2 of | ||
// https://lamport.azurewebsites.net/tla/proving-safety.pdf | ||
module teachingConcurrencyN { | ||
// N processes (numbered from 0 to N-1). | ||
const NUM_PROCS: int | ||
assume nonEmpty = NUM_PROCS >= 1 | ||
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var x: List[int] | ||
var y: List[int] | ||
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// Set of processes yet to be executed. | ||
var pendingProcs: Set[int] | ||
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// Initial state: | ||
// - x and y zeroed for all processes | ||
// - all processes pending scheduling | ||
action init = all { | ||
x' = zeroes(NUM_PROCS), | ||
y' = zeroes(NUM_PROCS), | ||
pendingProcs' = 0.to(NUM_PROCS-1), | ||
} | ||
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// Stepping action: randomly pick a pending process (if | ||
// any) and execute it. Otherwise stutter forever, as | ||
// every process has stopped. | ||
action step = { | ||
if (termination) { | ||
allStopped | ||
} else { | ||
processUpdate | ||
} | ||
} | ||
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// Execute the algorithm for a pending process. | ||
action processUpdate = { | ||
nondet proc = pendingProcs.oneOf() | ||
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val nextX = x.replaceAt(proc, 1) | ||
val index = circularIndex(proc, NUM_PROCS) | ||
val yValue = nextX[index] | ||
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all { | ||
x' = nextX, | ||
y' = y.replaceAt(proc, yValue), | ||
pendingProcs' = pendingProcs.exclude(Set(proc)), | ||
} | ||
} | ||
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// All processes have stopped. | ||
action allStopped = all { | ||
x' = x, | ||
y' = y, | ||
pendingProcs' = pendingProcs, | ||
} | ||
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// Return a List of 0s, with the given length. | ||
pure def zeroes(len: int): List[int] = { | ||
0.to(len-1).fold(List(), (l, _) => l.append(0)) | ||
} | ||
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// Calculate the circular index `(i-1) mod N`. | ||
pure def circularIndex(i: int, N: int): int = { | ||
if (i == 0) { | ||
N-1 | ||
} else { | ||
i-1 | ||
} | ||
} | ||
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// Correctness invariant: (after every process has | ||
// stopped) y[i] == 1 for at least one process. | ||
val yContainsOne = { | ||
y.select(v => v == 1).length() >= 1 | ||
} | ||
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// Termination invariant: all processes have stopped | ||
// (i.e. none is pending scheduling). | ||
val termination = { | ||
pendingProcs.size() == 0 | ||
} | ||
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temporal correct = eventually(termination and yContainsOne) | ||
} |
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// Generated by genversion. | ||
export const version = '0.21.1' | ||
export const version = '0.21.2' |
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