Re: Volatile vs Non-Volatile Spin Locks on SMP.

[Robert Hancock <hancockr@shaw.ca>]

multisyncfe991@hotmail.com wrote:
> Hello,
> 
> By using volatile keyword for spin lock defined by in spinlock_t, it 
> seems Linux choose to always
> reload the value of spin locks from cache instead of using the content 
> from registers. This may be
> helpful for synchronization between multithreads in a single CPU.
> 
> I use two Xeon cpus with HyperThreading being disabled on both cpus. I 
> think the MESI
> protocol will enforce the cache coherency and update the spin lock value 
> automatically between
> these two cpus. So maybe we don't need to use the volatile any more, right?

The value must always be loaded from memory. If the value is cached in a 
register it will not update when another CPU changes it.

-- 
Robert Hancock      Saskatoon, SK, Canada
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