Q:

How can I avoid preemption of my thread in user mode

I have a simple chunk of deterministic work that only takes thirteen machine instructions to complete. Because the first instruction takes a homemade semaphore (spinlock) and the last instruction releases it, I am safe from all of the other threads running on the other cores as they are attempting to take and give the same semaphore. 
The problem arises when some thread interrupts a thread holding the semaphore before it can finish its "critical section". Worst case the interruption kills the thread while holding the semaphore or as can happen one of the threads normally competing for the semaphore branches out into code that can generate the interrupt causing a deadlock. 
I don't have a way synchronizing with these other threads when they branch into those parts of the code I can't control. I think I need to disable interrupts like I used to do in my old VxWorks days when I was running in kernel mode. Its always thirteen instructions and I am always completely safe if I can get all thirteen instructions done before I have to honor an interrupt. Oh and it is all my own internal data, other that the homemade semaphore there is nothing that locks anything else up.
I have read several answers that I think are close. Most have to do with Critical Section calls on the Windows API (wrong OS but maybe the right concept). Most of the wrong solutions assume that I can get all of the offending threads to use a mutex that I create with the pthread libraries.
I need this solution in C/C++ on Linux and Solaris.
Johnny Crash's question is very close
prevent linux thread from being interrupted by scheduler
KermitG also
Can I prevent a Linux user space pthread yielding in critical code?
Thanks for your consideration.

A:

You may not prevent preemption of a user-mode thread. Critical sections (and all other sync objects) prevent collisions of your threads, however they by no means prevent them from preemption by the OS.
If your other threads branch into something on timeout, whereas that something may lead to a deadlock - you have a design problem.
A correct design should be the most pessimistic: preemption may occur everywhere for indeterminate time.

