Comment by MaxBarraclough
15 hours 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...
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.
1 reply →
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.
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.