__asm__ __volatile__ usage in linux kernel source

__asm__ is a gcc extension of permitting assembly language statements to be entered nested within your C code

If our assembly statement must execute where we put it, (i.e. must not be moved out of a loop as an optimization), put the keyword volatile after asm and before the ()’s. So to keep it from moving, deleting and all, we declare it as
asm volatile ( ... : ... : ... : ...);
Use __volatile__ when we have to be verymuch careful.
If our assembly is just for doing some calculations and doesn’t have any side effects, it’s better not to use the keyword volatile. Avoiding it helps gcc in optimizing the code and making it more beautiful.