Re: Question from Russells Spinlocks

[Brian Gerst]

Imanpreet Singh Arora wrote:
> Hello there,
>
>    I was reading Russell's guide on spinlocks, and I have some questions 
> regarding it.
>
>
>    Question-->    Russell says that in case of non-SMP machines 
> spinlocks don't exist _at_ALL_. Well they should do something don't they 
> like disable interrupts and premptations. I checked linux/spinlock well 
> they DO NOT do anything atleast not when DEBUG_SPINLOCKS == 0. My 
> understanding is that since they aren't used anywhere outside kernel and 
> drivers(?), they can't be prempted. At least that is what I have read.

On UP kernels, the spinlock operations that include interrupt 
manipulations still affect interrupts.  It's only the actual lock 
portion that becomes a no-op.

> What does the comment about gcc while defining atomic_t signify?
>             --> What about the comment about the limit of 24 bits on 
> atomic_t?                a)    Atomic operations on integers are 
> guranteed only if there value can be stored in 24 bits.
>             b)    Atomic operations are guranteed only if the pointer 
> has 8 MSbits set zero.

In 2.6, the 24-bit limit is no longer valid.  atomic_t variables are a 
full 32 bits on all arches now.

--
				Brian Gerst

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