Re: Question from Russells Spinlocks

[Parag Warudkar]

Anton Altaparmakov wrote:

> On Thu, 9 Dec 2004 kernel-stuff@xxxxxxxxxxx wrote:
>  
>
> > Spinlocks are used in situations where multiple threads contend for a 
> > lock and they can possibly run on more than one CPU. Example - Thread A 
> > is executing on CPU-A, Thread B in executing on CPU-B. They contend for 
> > a lock L1.  A acquires the lock first. B tries (on CPU-B) to acquire the 
> > lock L1 and finds it is not free - so it just spins (executes a no-op 
> > kind of loop ) until Thread A relinquishes the lock L1. Spinlocks are 
> > used in cases where the operation performed under a lock is short one - 
> > takes very less time. In these type of cases, spinning is less costlier 
> > than sleeping which involves scheduler overhead. So if we take out CPU-B 
> > from the above equation - there is no chance for Thread B to execute to 
> > contend for lock L1 without thread A going to sleep. That's why 
> > spinlocks are useless on 1 CPU machine.
> >    
>
> Your last sentence is incorrect.  Spinlocks on 1 CPU machines still need 
> to disable preemption (assuming preemption is compiled in obviously, if 
> not then indeed you are right).  Otherwise preemption could take place in 
> the middle of a data manipulation and you would still have the same race 
> as you described with two cpus working concurrently.  Except that with 
> preemption it is only logical concurrence not actual physical concurrence.
>  
I didn't take pre-emption into account. Although I would not have 
readily thought about the need to disable Pre-emption even if I had 
considered it  :-) So thanks for the correction!

Parag
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