Re: [PATCHv2] bitops: add _local bitops

From: Avi Kivity
Date: Sun May 13 2012 - 06:13:50 EST

On 05/10/2012 10:04 PM, Christoph Lameter wrote:
> On Wed, 9 May 2012, Michael S. Tsirkin wrote:
> > kvm needs to update some hypervisor variables atomically
> > in a sense that the operation can't be interrupted
> > in the middle. However the hypervisor always runs
> > on the same CPU so it does not need any memory
> > barrier or lock prefix.
> >
> > Add _local bitops for this purpose: define them
> > as non-atomics for x86 and (for now) atomics for
> > everyone else.
> Have you tried to use the this_cpu_ops for that purpose? They create the
> per cpu atomic instructions that you want without a lock prefix and can
> also relocate the per cpu pointer to the correct processor via a
> segment register prefix.
> There are no bit operations provided right now but those can either be
> improvised using this_cpu_cmpxchg or added.

this_cpu_xchg() should be sufficient, since only bit zero has any
meaning in our use case (so xchg with zero is equivalent to

error compiling committee.c: too many arguments to function

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