My understanding is that Rust prevents data races, but not all race conditions. You can still get a logical race where operations interleave in unexpected ways. Rust can’t detect that, because it’s not a memory-safety issue.
So you can still get deadlocks, starvation, lost wakeups, ordering bugs, etc., but Rust gives you:
- No data races
- No unsynchronized aliasing of mutable data
- Thread safety enforced through type system (Send/Sync)
This fits quite naturally in Rust. You can let your mutex own the pair: locking a `Mutex<(u32, u32)>` gives you a guard that lets you access both elements of the pair. Very often this will be a named `Mutex<MyStruct>` instead, but a tuple works just as well.
Because rust guarantees you won't have multiple exclusive (and thus mutable refs), you won't have a specific class of race conditions.
Sometimes however, these programs are very strict, and you need to relax these guarantees. To handle those cases, there are structures that can give you the same shared/exclusive references and borrowing rules (ie single exclusive, many shared refs) but at runtime. Meaning that you have an object, which you can reference (borrow) in multiple locations, however, if you have an active shared reference, you can't get an exclusive reference as the program will (by design) panic, and if you have an active exclusive reference, you can't get any more references.
This however isn't sufficient for multithreaded applications. That is sufficient when you have lots of pieces of memory referencing the same object in a single thread. For multi-threaded programs, we have RwLocks.
Rust approach to shared memory is in-place mutation guarded by locks. This approach is old and well-know, and has known problems: deadlocks, lock contention, etc. Rust specifically encourages coarse-granular locks by design, so lock contention problem is very pressing.
There are other approaches to shared memory, like ML-style mutable pointers to immutable data (perfected in Clojure) and actors. Rust has nothing to do with them, and as far as I understand the core choices made by the language make implementing them very problematic.
Would you mind elaborating on this? At least off the top of my head a mut Arc<T> seems like it should suffice for a mutable pointer to immutable data, and it's not obvious to me what about actors makes implementing them in Rust very problematic.
Logical race conditions and deadlocks can still happen.
Of course the borrow checker and when you use lifetimes can be complex to learn, especially if you’re coming from GC-land, just the language syntax isn’t really that weird.
Rust data types can be "Send" (can be moved to another thread) and "Sync" (multiple threads can access them at the same time). Everything else is derived from these properties (structs are Send if their fields are Send. Wrapping non-Sync data in a Mutex makes it Sync, thread::spawn() requires Send args, etc.)
Rust doesn't even reason about thread-safety of functions themselves, only the data they access, and that is sufficient if globals are required to be "Sync".