Re: mutex vs cache coherency protocol(for multiprocessor )

[Chris Snook <csnook@redhat.com>]

Xu Yang wrote:
> Hello everyone,
>
> Just got a rough question in my head.
>
> don't know whether anyone interested .
>
> mutex vs cache coherency protocol(for multiprocessor)
>
> both of these two can be used to protect shared resource in the memory.
>
> are both of them necessary?
>
> for example:
>
> in a multiprocessor system, if there is only mutex no cache coherency.
> obviously this would cause problem.
>
> what about there is no mutex mechanism, only cache coherency protocol
> in multiprocessor system? after consideration, I found this also could
> casue problem, when the processors are multithreading processors,
> which means more than one threads can be running on one processor. in
> this case if we only have cache coherency and no mutex, this would
> cause problem. because all the threads running on one processor share
> one cache, the cache coherency protocol can not be functioning
> anymore. the shrared resource could be crashed by different threads.
>
> then if all the processors in the multiprocessor system are sigle
> thread processor, only one thread can be running one one processor. is
> it ok, if we only have cache coherency protocol ,no mutex mechanism?
>
> anyone has any idea? all the comments are welcome and appreciated,
> including criticism.

Cache coherency is necessary for SMP locking primitives (and thus Linux SMP 
support), but it is hardly sufficient.  Take a look at all the exciting inline 
assembly in include/asm/ for spinlocks, atomic operations, etc.

	-- Chris
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