Context switch internals

Question

I want to learn and fill gaps in my knowledge with the help of this question.

So, a user is running a thread (kernel-level) and it now calls yield (a sy

Answer 1

What you missed during step 2 is that the stack gets switched from a thread's user-level stack (where you pushed args) to a thread's protected-level stack. The current context of the thread interrupted by the syscall is actually saved on this protected stack. Inside the ISR and just before entering the kernel, this protected-stack is again switched to the kernel stack you are talking about. Once inside the kernel, kernel functions such as scheduler's functions eventually use the kernel-stack. Later on, a thread gets elected by the scheduler and the system returns to the ISR, it switchs back from the kernel stack to the newly elected (or the former if no higher priority thread is active) thread's protected-level stack, wich eventually contains the new thread context. Therefore the context is restored from this stack by code automatically (depending on the underlying architecture). Finally, a special instruction restores the latest touchy resgisters such as the stack pointer and the instruction pointer. Back in the userland...

To sum-up, a thread has (generally) two stacks, and the kernel itself has one. The kernel stack gets wiped at the end of each kernel entering. It's interesting to point out that since 2.6, the kernel itself gets threaded for some processing, therefore a kernel-thread has its own protected-level stack beside the general kernel-stack.

Some ressources:

• 3.3.3 Performing the Process Switch of Understanding the Linux Kernel, O'Reilly
• 5.12.1 Exception- or Interrupt-Handler Procedures of the Intel's manual 3A (sysprogramming). Chapter number may vary from edition to other, thus a lookup on "Stack Usage on Transfers to Interrupt and Exception-Handling Routines" should get you to the good one.

Hope this help!