Re: Question from Russells Spinlocks

[Parag Warudkar <kernel-stuff@comcast.net>]

Anton Altaparmakov wrote:

>On Thu, 9 Dec 2004 kernel-stuff@comcast.net 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