CS372: Solutions for Homework 5

Problem 1

Consider a uniprocessor kernel that user programs can trap into using system calls. The kernel receives and handles interrupt requests from I/O devices. Would there be any need for critical sections within that kernel?

Yes. Assume a user program enters the kernel through a trap. While running the operating system code, the machine receives an interrupt. Now, the interrupt handler may modify global data structure that the kernel code was trying to modify. Therefore, while there is only one thread that runs inside the kernel at any given time, the kernel may not be re-entrant if access to global data structures is not protected through the use of appropriate mutexes.