Why is spinlock no-op in Linux kernel (non-SMP)?

Question

I've been reading this for quite some time but doesn't make sense to me.. Probably because I'm new to all this and still don't understand few kernel concepts.

This was what i came up with (no error or NULL handing, it's just for the sake of the question):

Kernel spinlocks are executed inside kernel threads, which is preemtive.

void spinlock_acquire(spinlock_t *spinlock)
{
  tryagain:
    while(spinlock->plock != UNLOCKED) ;
    context_switch_block;
    if(spinlock->plock != UNLOCKED) {
        context_switch_unblock;
        goto tryagain;
    }
    spinlock_lock(spinlock, current_thread);
    context_switch_unblock;
}

Answer 1

Before Linux was a preemptive kernel, spinlocks on UP were basically no-ops. Once the kernel was made preemptive, a call to preempt_disable() was added to spinlocks.

So it goes more or less like this:

• You want to protect against some conflicting CPU, use some kind of spinlock.
• You want to protect against a conflicting softirq, tasklet,... use spin_lock_bh, which disables softirqs, tasklets, etc... (bh is for historical name, it comes from "bottom half").
• You want to protect against a conflicting hardware interrupt use spin_lock_irq*, which disables hardware interrupts.
• All spinlocks protect against preemption.
• On a UP kernel, spinlocks don't take a real spinlock (since there are no conflicting CPUs, and we cannot be preempted, and there are spinlock variants for dealing with hardirqs, softirqs,...).
• On a UP machine with an SMP kernel, spinlocks may be turned into nops.
• Even on a UP kernel with preemption disabled, spinlocks may have code for spinlock debugging, if it is enabled.

Answer 2

Spin lock is unnecessary on non-SMP. Since a spin lock disabled interrupts, it is not possible to anyone else to have the lock at that point. Once thread A disabled interrupts, is is not possible for thread B to try and acquire the same lock, as there is nothing that can cause A to lose the CPU in favor of B. As such, all spin lock does on non-SMP is, well, nothing (except if you ask it to disable interrupts).

Shachar