Why Doesn't Go's `sync.Mutex` Support Recursive Locking?

Recursive Locking

Go's sync.Mutex provides a robust synchronization mechanism for concurrent programming. However, it lacks support for recursive locking, which can pose a challenge when dealing with certain scenarios.

Why Not Implement Recursive Locks?

While it may seem logical to implement recursive locks in Go, it's important to consider the underlying principles of synchronization. As Russ Cox from the Go development team eloquently puts it, recursive mutexes "do not protect invariants."

Mutex primitives are designed to enforce thread-safety and ensure data consistency. When a mutex is held, it guarantees that the protected data structures are in a valid state. Recursive locks, however, break this guarantee by allowing threads to acquire the same mutex multiple times within a single execution path. This can lead to incorrect or undefined behavior, making it inherently challenging to maintain data integrity.

Alternative Solutions

Instead of resorting to recursive locks, it's recommended to redesign code to avoid their need in the first place. A more robust and scalable approach is to separate protected code into small, atomic tasks that can be executed outside the scope of any mutex. This ensures that the protected data remains consistent throughout the entire code execution.

Case in Point

Consider the example provided in Russ Cox's response:

func F() {
    mu.Lock()
    ... do some stuff ...
    G()
    ... do some more stuff ...
    mu.Unlock()
}

func G() {
    mu.Lock()
    ... do some stuff ...
    mu.Unlock()
}

This code demonstrates the potential pitfalls of using recursive locks. If F breaks the invariants it's responsible for protecting before calling G, G will continue to operate on the inconsistent data, leading to erroneous results.

To resolve this issue, a more appropriate approach would be to define a separate helper function g that does not require the mutex protection:

// To be called with mu already held.
func g() {
    ... do some stuff ...
}

func G() {
    mu.Lock()
    g()
    mu.Unlock()
}

This approach ensures that G always operates on the protected data when it's in a consistent state, effectively avoiding the risks associated with recursive locks.

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