Hard to say without knowing what you mean by "if two tasks try to access a variable concurrently it gets marked atomic and they contend for access." People who have worked in depth with multithreaded programs know that there be dragons here, and so unless you've proved your code correct for all cases, the assumption will be that it's broken. Do you handle the case that Thriqon brings up safely, for example? Do you take all locks in a consistent well-defined order? (If you don't, you're inviting deadlock.) What counts as a variable? If you mutate an array element, do you lock the whole array or just that one object?

These issues are why Python has a GIL and why Java has separate "synchronized(monitor) { ... }" blocks, and Java programmers still get it wrong all the time.