linux-kernel - Re: mutex vs cache coherency protocol(for multiprocessor )

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Date:	Tue, 04 Sep 2007 08:59:41 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Xu Yang <risingsunxy@...glemail.com>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: mutex vs cache coherency protocol(for multiprocessor )

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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