Avoiding multiple lock dead-locks by memory addresses

Sometimes you need to lock several resources. If you don’t take great care you are likely to get yourself into dead-locks. A simple example with just two lockable resources A and B:

function foo {
  lock A;
  lock B;
  // Do something with A and B
  unlock A;
  unlock B;
}

function bar {
  lock B;
  lock A;
  // Do something different with A and B
  unlock B;
  unlock A;
}

When foo and bar are called at about the same time then there is the change that foo locks A and bar locks B which will make bar wait on foo‘s lock on A and vice versa.

Solution: fixed order on memory address
The simplest way to get rid of the deadlock is to always try to acquire a lock on A before on B. A generic solution would be to always lock the resource with the lowest memory address first.

This only works when memory addresses are fixed or that there is an otherwise fixed order-able property.