How to avoid deadlocks?

Question

When using multiple threads, shared memory needs to be locked by critical sections. However, using critical sections causes potential deadlocks. How can they be avoided?

Answer 1

When I work in C++, the following works for me:

1. all public methods (excluding ctor and dtor) of a threadsafe class lock

2. private methods cannot call public methods

It's not a general deadlock avoidance method.

Answer 2

The Related list to the right on this page contains a few links that provides interesting information on the topic.

In addition to that list, there are many other SO questions discussing the topic, such as

• Threading Best Practices
• Why is lock(this) {…} bad?
• What are common reasons for deadlocks?

...and many more

Answer 3

You must code multi-thread programs very carefully. There's no short-cut, you must understand the flow of your program, otherwise you'll be doomed.

Answer 4

One way is by using a non-blocking locking function. As an example, in rust You could use std::sync::Mutex::try_lock instead of std::sync::Mutex::lock.

So so if you have this example code:

fn transfer(tx: &Mutex<i32>, rx: &Mutex<i32>, amount: i32) -> () {
    let mut tx = tx.lock().unwrap();
    let mut rx = rx.lock().unwrap();

    *tx -= amount;
    *rx += amount;
}

You could instead do something like this:

fn transfer(tx: &Mutex<i32>, rx: &Mutex<i32>, amount: i32) -> () {
    loop {
        // Attempt to lock both mutexes
        let mut tx = tx.try_lock();
        let mut rx = rx.try_lock();

        // If both locks were successfull,
        // i.e. if they currently are not
        // locked by an other thread
        if let Ok(ref mut tx) = tx {
            if let Ok(ref mut rx) = rx {
                // Perform the operations needed on
                // the values inside the mutexes
                **tx -= amount;
                **rx += amount;

                // Exit the loop
                break;
            }
        }
        // If at least one of the locks were
        // not successful, restart the loop
        // and try locking the values again.

        // You may also want to sleep the thread
        // here for a short period if You think that
        // the mutexes might be locked for a while.
    }
}

Answer 5

You can avoid critical sections by using message passing instead (synchronous and asynchronous calls). When using synchronous calls, you still have to make sure not to make a circular call, in which thread A asks thread B a question, and B needs to ask A a question to be able to respond.

Another option is to make asynchronous calls instead. However, it is more difficult to get return values.

Note: Indeed, a message passing system is implemented using a critical section that locks the call queue, but it is abstracted away.

Answer 6

One way is to use a hierarchy of critical sections. If you ensure that a parent critical section is never entered within one of its children, deadlocks cannot happen. The difficulty is to enforce this hierarchy.