Atomic action - mutex

Question

I heard that there is something known as "atomic action" which is faster then using a mutex with critical section. Does somebody know what is it, and how do I use it?

Answer 1

An atomic operation is an operation where the CPU reads and writes memory during the same bus access, this prevents other CPUs or system devices from modifying the memory simultaneously. E.g. a "test and set" operation, which could do "read memory at location X, if it is 0 set it to 1, return an indicator telling whether the value was set" without any chance of simultaneous access.

Wikipedia's http://en.wikipedia.org/wiki/Linearizability describes atomic operations.

If you're on windows, have a look at e.g. InterlockedTestExchange or InterlockedIncrement which are wrappers for atomic operations.

EDIT: Sample usage

A spinlock could be implemented using a test-and-set atomic operation:

while (test_and_set(&x) == 1) ;

This will keep looping until the current thread is the one that sets x to 1. If all other threads treat x in the same way its effect is the same as a mutex.