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[docs] add danger admonition for Threads.threadid #3909

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Jan 8, 2025
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[docs] add danger admonition for Threads.threadid #3909

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13 changes: 13 additions & 0 deletions docs/src/tutorials/algorithms/parallelism.md
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Expand Up @@ -74,6 +74,19 @@
3
````

!!! danger
The `Threads.threadid()` that a task runs on may change during execution.
Therefore, it is not safe to use `Threads.threadid()` to index into, say, a
vector of buffer or stateful objects. As an example, do not do:
```julia
x = rand(Threads.nthreads())
Threads.@threads for i in 1:Threads.nthreads()
x[Threads.threadid()] *= 2 # Danger! This use of threadid is not safe
end
```
For more information, read
[PSA: Thread-local state is no longer recommended](https://julialang.org/blog/2023/07/PSA-dont-use-threadid/).

Check warning on line 88 in docs/src/tutorials/algorithms/parallelism.md

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[vale] reported by reviewdog 🐶
[Google.Colons] ': T' should be in lowercase.

Raw Output:
{"message": "[Google.Colons] ': T' should be in lowercase.", "location": {"path": "docs/src/tutorials/algorithms/parallelism.md", "range": {"start": {"line": 88, "column": 9}}}, "severity": "WARNING"}

### Thread safety

When working with threads, you must avoid race conditions, in which two
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