CODEWITHSUNDEEP
linuxkernelspinlock
sgupta
sgupta

Reputation: 1272

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

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;
}

Upvotes: 1

Views: 3318

Answers (2)

ninjalj
ninjalj

Reputation: 43688

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.

Upvotes: 9

Shachar Shemesh
Shachar Shemesh

Reputation: 8563

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

Upvotes: 2

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