Thanks for the reply.

Letting context switches from only main task function seems quite restricted. And I'm trying to consider the preemptive approach also.

For all tasks using reentrant functions, do not I need seperate reentrant stack area? And also, I need seperate hw-stack area for all tasks? Right?

Otherwise, one can corrupt others' data on any stack.

How can I have different reentrant stack pointer/area and system stack pointer/area? (If it is OK to switch context on any level of function call)

Saving all the stack content to another place does not seem reasonable.