What is reentrant kernel:
As the name suggests, a reentrant kernel is the one which allows
multiple processes to be executing in the kernel mode at any given
point of time and that too without causing any consistency problems
among the kernel data structures.
What is kernel preemption:
Kernel preemption is a method used mainly in monolithic and hybrid
kernels where all or most device drivers are run in kernel space,
whereby the scheduler is permitted to forcibly perform a context
switch (i.e. preemptively schedule; on behalf of a runnable and higher
priority process) on a driver or other part of the kernel during its
execution, rather than co-operatively waiting for the driver or kernel
function (such as a system call) to complete its execution and return
control of the processor to the scheduler.
Can I imagine a preemptive kernel which is not reentrant? Hardly, but I can. Let's consider an example: some thread performs a system call. While entering a kernel it takes a big kernel lock and forbids all interrupt except scheduler timer irq. After that this thread is preempted in kernel by a scheduler. Now we may switch to another userspace thread. This process do some work in userspace and after that enters kernel, take big kernel lock and sleeps and so on. In practice looks like this solution can't be implemented, because of huge latency due to forbidding interrupts on a big time intervals.
Can I imagine reentrant kernel which is not preemptive? Why not? Just use cooperative preemption in kernel. Thread 1 enters kernel and calls thread_yield() after some time. Thread 2 enters kernel do it's own work maybe call another thread_yield maybe not. There is nothing special here.
As for linux kernel it is absolutely reentrant, the kernel preemption may be configured by CONFIG_PREEMPT. Also voluntary preemption is possible and many other different options.
