Software

Software-defined mutual exclusion implementations are all based on busy-waiting. An example is Dekker's algorithm, which defines a system in which two processes can synchronize, employing busy-wait to wait for the other process to leave the critical section.

The pseudocode for this algorithm is as follows:

    variables
        wants_to_enter : array of 2 booleans
        turn : integer

    wants_to_enter[0] ← false
    wants_to_enter[1] ← false
    turn ← 0 // or 1

p0:
    wants_to_enter[0] ← true
    while wants_to_enter[1] {
        if turn ≠ 0 {
            wants_to_enter[0] ← false
            while turn ≠ 0 {
                // busy wait
            }
            wants_to_enter[0] ← true
        }
    }
    // critical section
    ...
    turn ← 1
    wants_to_enter[0] ← false
    // remainder section

p1:
    wants_to_enter[1] ← true
    while wants_to_enter[0] {
        if turn ≠ 1 {
            wants_to_enter[1] ← false
            while turn ≠ 1 {
                // busy wait
            }
            wants_to_enter[1] ← true
        }
    }
    // critical section
    ...
    turn ← 0
    wants_to_enter[1] ← false
    // remainder section

(Referenced from: https://en.wikipedia.org/wiki/Dekker's_algorithm)

In this preceding algorithm, processes indicate the intent to enter a critical section, checking whether it's their turn (using the process ID), then setting their intent to enter the section to false after they have entered it. Only once a process has set its intent to enter to true again will it enter the critical section again. If it wishes to enter, but turn does not match its process ID, it'll busy-wait until the condition becomes true.

A major disadvantage of software-based mutual exclusion algorithms is that they only work if out-of-order (OoO) execution of code is disabled. OoO means that the hardware actively reorders incoming instructions in order to optimize their execution, thus changing their order. Since these algorithms require that various steps are executed in order, they no longer work on OoO processors.