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Single word read and write is atomic on x86 SMP system. So no need for locking the statement a=b. Its atomic by nature. Even if you use spinlock to protect a=b, any thread reading a will either get previous value of a or new value(ie b). And without using any lock for a=b, will also have same effect on any thread reading value of a. So no need to lock a=b statement

disable all interrutp when this statement get executed in case of uniprocessor.

As per ARM , you can use swp instruction which will hold/lock the bus and store the value of b in a.
Cant say about x86.

if you allow oops, Put the above methods by Dinesh, chinmoy or sem inside a constructor, that will make a=b sweet rather than enabling and disabling interrupts everytime you do an assignement. Further RAII will take care of enabling the interrupts if something goes wrong

a and b could be grobal variables and multiple processes could be modifying a or b at any time without using a lock.

In such case

.........

b = 1;

<======== (some process changed b here. Let b = 100)

a = b; (we think a =1, but a = 100 now!)

.....................


Use atomic_set(atomic_t *v, int i);

this atomic family is implemented in assembly lanuage since architecture depended. They are atomical.

You must disable all the interrupts if the variable can be referred in ISR, moreover, you maybe need disable preempt, there is other process maybe use the variable.