[Xen-devel] Re: [RFC PATCH 05/35] Add sync bitops

[Christoph Lameter <clameter@xxxxxxx>]

On Tue, 9 May 2006, Chris Wright wrote:

> Add "always lock'd" implementations of set_bit, clear_bit and
> change_bit and the corresponding test_and_ functions.  Also add
> "always lock'd" implementation of cmpxchg.  These give guaranteed
> strong synchronisation and are required for non-SMP kernels running on
> an SMP hypervisor.

Could you explain why this is done and what is exactly meant with "always 
looked"? Wh the performance impact?

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