Describe how to prevent deadlocks when using multiple `Mutex`es in Rust. What is the most common strategy?
Go & Rust interview question for Advanced practice.
Answer
Deadlocks in Rust when using multiple Mutexes typically arise from a circular wait: Thread A locks Mutex 1 and then tries to lock Mutex 2, while Thread B locks Mutex 2 and then tries to lock Mutex 1. If their execution interleaves, both threads will be stuck waiting for a lock held by the other. The most common and effective strategy to prevent this is to enforce a strict global lock ordering. 1. Establish an Order: Assign a fixed, arbitrary order to all mutexes in your program. For example, you might decide that Mutex A must always be locked before Mutex B, which must be locked before Mutex C, and so on. 2. Enforce the Order: In your code, ensure that any thread that needs to acquire multiple locks does so strictly according to this pre-defined order. If a thread holds a lock on Mutex B and needs to acquire a lock on Mutex A, it must first release its lock on B, then acquire A, then re-acquire B. By ensuring that all threads acquire locks in the same hierarchical order, you make a circular wait impossible, thus preventing deadlocks. Other less common strategies include using trylock, which attempts to acquire a lock but returns immediately without blocking if it's unavailable, allowing the thread to do other work or release its held locks and try again later.
Explanation
Rust's ownership system and borrow checker significantly reduce the risk of data races, but logical errors like deadlocks are still possible and must be handled by the programmer.