spin until lock is acquired

Question

I am reading some material on test and set instructions from Wiki(https://en.wikipedia.org/wiki/Test-and-set)

What i understand is most CPUs support a special instruction "test and set" to achieve mutual exclusion. However I do not understand is the it talks about CPU spinning to acquire a lock. Can this be implemented on single core systems. If yes then spinning to acquire the lock do not make sense to me. Or may be I am missing the point where test ans set instruction is only possible on multiple core systems?

boolean lock = false
function Critical(){
    while TestAndSet(lock)
        skip // **spin until lock is acquired**
    critical section // only one process can be in this section at a time
    lock = false // release lock when finished with the critical section
}

Thanks in advance for your feedback.

score 0 · Accepted Answer · answered Jul 08 '15 at 08:07

0

On preemptive multithreading systems, other processes are allowed to run between TestAndSet==false and skip as well as between skip and TestAndSet. So it works on single core systems as well.

answered Jul 08 '15 at 08:07

D.R.

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Thanks D.R . But doesn't that mean its not atomic any more? – Zoolander Jul 08 '15 at 08:26
No, the important thing is that the `TestAndSet` operation is atomic, not that the whole while loop is atomic. – D.R. Jul 08 '15 at 08:27
I see. Thanks again. – Zoolander Jul 08 '15 at 08:28
It works but VERY badly. If the priority of the caller is very high, it could effectively sieze up the entire box. – Martin James Jul 08 '15 at 12:27

spin until lock is acquired

1 Answers1