Comment by MaxBarraclough
6 months ago
Neat to see sleep calls artificially introduced to reliably recreate the deadlock. [0]
Looks like fixing the underlying bug is still in-progress, [1] I wonder how many lines of code it will take.
[0] https://github.com/aoli-al/jdk/commit/625420ba82d2b0ebac24d9...
without reworking of the code all these checks of the executor and queue state and queue manipulations have to be under a mutex, and that is just a few lines.
Bugs like these are pervasive in languages like Java that give no protection against even the most basic race condition causes. It’s nearly impossible to write reliable concurrent code. Freya only helps if you actually use it to test everything which is not realistic. I am convinced, after my last year long struggle to get a highly concurrent Java (actually Kotlin but Kotlin does not add much to help) module at work, that we should only use languages that provide safe concurrency models, like Erlang/Elixir and Rust, or actor-like like Dart and JavaScript, where concurrency is required.
What is a safe concurrency model? Like, actors can trivially deadlock/livelock, they are no panacea at all, and are trivial to recreate (there are a million java implementations)
You make it sound like there is some modern development superseding what java has, but that's absolutely not the case.
Like even rust is just pretty much a no-overhead `synchronized` on top of an object. It is necessary there, because data races are a fundamental memory safety issue, but Java is immune to that (it has "safe" data races). Logical bugs can trivially happen in either case - as an easy example even if all your fields are atomically mutated, the whole object may not make sense in certain states, like a date with February the 31st. Rust does nothing against such, and concurrent data structures have ample grounds for realistic examples of the above.
> What is a safe concurrency model?
STM.
The terms 'atomic', 'thread-safe', and 'concurrent' collections are thrown around too loosely for application programmers IMO, for exactly your example above.
In other scenarios, 'atomics' refer to the ability to do one thing atomically. With STM, you can do two or more things atomically.
Likewise with 'thread-safe'. Thread-safe seems to indicate that the object won't break internally in the presence of multiple threads, which is too low of a bar to clear if your goal is to write an actually thread-safe application out of so-called 'thread-safe' parts.
STM has actual concurrent data structures, where you can write straight-line code like 'if this collection has at least 5 elements, then pop one'.
I don't think the Feb 31 example is that fair though, because if you want to construct a representation of Feb 31, who's going to stop you? And if you don't want to, plain old static types is the solution.
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> the whole object may not make sense in certain states
"Make invalid states unrepresentable" - it's bad design that February the 31st is a thing in your data structure when that's invalid. You can't always avoid this, but it's appalling how bad most people's data structures are.
C's stdlib provides a tm structure in which day of the week is stored in a signed 32-bit integer. You know, for when it's the negative two billionth day of the week...
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> Like, actors can trivially deadlock/livelock,
Oh my ... you never seen a proper Actor language, have you?
Have a look at Erlang and Pony, for starters. It will open your mind.
This in particular is great: https://www.ponylang.io/discover/what-makes-pony-different/#...
> Pony doesn’t have locks nor atomic operations or anything like that. Instead, the type system ensures at compile time that your concurrent program can never have data races. So you can write highly concurrent code and never get it wrong.
This is what I am talking about.
> You make it sound like there is some modern development superseding what java has, but that's absolutely not the case.
Both Actor-model languages and Rust (through a surprisingly different path: tracking aliases and lifetimes) do something that's impossible in Java (and most languages): prevent data races due to improper locking (as mentioned above, if your language even has locks and it doesn't make them safe like Rust does, you know you're going to have a really hard time. actor-languages just eliminate locks, and "manual concurrency", completely). Other kinds of races are still possible, but preventing data races go a very, very long way to making concurrency safe and easy.
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Race conditions are generally solved with algorithms, not the language. For example, defining a total ordering on locks and only acquiring locks in that order to prevent deadlock.
I guess there there are language features like co-routines/co-operative multi-tasking that make certain algorithms possible, but nothing about Java prevents implementing sound concurrency algorithms in general.
> Race conditions are generally solved with algorithms, not the language. For example, defining a total ordering on locks
You wouldn't make that claim if your language didn't have locks.
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