Can a user thread directly enter kernel w/o a kernel thread being called?

Question

Can a user thread directly enter kernel w/o a kernel thread being called ? I mean, a user space enter kernel via an exception, w/o calling any kernel thread, but run in kernel code. Thanks !

Answer 1

Yes, they can. User threads enter the kernel all the time for system calls, interrupts and exceptions. While the kernel is handling one of these it is running in kernel mode in the context of the user thread (and uses that thread's kernel stack).

In fact, a "kernel thread" is defined as a scheduled task that never exits the kernel.