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Determinant and Inverse return wrong values when called from more than one thread

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#40 0 comentarios 0 reacciones 0 asignados Ver en GitHub

@Rafael-SOWNet ya está trabajando en esto.

Desde el 9/9/2026.

  • #41 de @Rafael-SOWNet — abierto

Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
35/100
Tipo de issue
Error
Claridad
Bien especificado
Estado de actividad
Estancado
Stack tecnológico
csharp
Área
performance

Línea de trabajo

Comienza con SquareMatrixFactory<T, TWrapper>.GetMatrix y ExpressionCompiler<T, TWrapper>.storage; después reproduce los fallos de determinante/inversa indicados con Parallel.For, incluyendo una caché fría. Se considera terminado cuando las operaciones concurrentes de matrices devuelven los valores verificados secuencialmente sin excepciones de colección ni corrupción de búferes compartidos.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Two process-wide statics are read and written with no synchronisation, so matrix operations called from more than one thread return wrong numbers. Nothing throws, nothing is malformed, and no result looks suspicious.

I found this from the other side: AngouriMath uses GenTensor for its symbolic matrices, and a determinant computed on a background thread was coming back numerically wrong. AngouriMath's issue is #1219.

1. The scratch-matrix pool

SquareMatrixFactory<T, TWrapper>.GetMatrix hands every caller the same GenTensor for a given size, and the caller then writes into it — Inversion.GetCofactorMatrix(t, temp, ...) fills the matrix it is given. Two threads taking a determinant or an inverse of the same size get the same buffer and overwrite each other's minors between the write and the read.

Sixty 5×5 int matrices, each computed once sequentially for truth and then rebuilt and recomputed under Parallel.For:

DeterminantLaplace    53 of 60 disagree
Adjoint               60 of 60 disagree

Around 10 ms, every run. This is not a rare interleaving, it is the normal case.

DeterminantGaussianSafeDivision is unaffected — it works on its own copy and never asks the pool for anything.

The lock inside GetMatrix does not help and is itself incomplete: the list is indexed after the lock is released, so a concurrent Add can reallocate the backing array underneath the reader.

2. The compiled-operation cache

ExpressionCompiler<T, TWrapper>.storage is a plain Dictionary, written by whichever thread first asks for a given (operation, rank, parallel). Dictionary<,> is documented as safe for concurrent readers only while nobody is writing.

Reached cold from several threads it fails with the runtime's own diagnostic:

System.InvalidOperationException: Operations that change non-concurrent collections must
have exclusive access. A concurrent update was performed on this collection and corrupted
its state. The collection's state is no longer correct.
...
The given key '(Subtraction, 1, False)' was not present in the dictionary.

Worth noting that this one hides easily. My first test computed its expected values sequentially and only then went parallel, which populated every key before the threads started — the cache holds a handful of entries, so a warm-up makes the defect invisible and the test passes against the broken code.

Fix

I have opened a PR. Both are small: [ThreadStatic] for the pool, since a scratch buffer has nothing to share between threads, and ConcurrentDictionary for the cache. The Laplace determinant comes out about 5% faster, because the pool is now taken once at the top of the recursion instead of re-checked and re-indexed at every one of its n! nodes.

Lenguaje dominante
C#
Estrellas
52
Forks
6
Sin PR fusionados en 30 d

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