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SubjectRe: [RFC patch] cmpxchg_double: remove local variables to get better performance
>>> On 02.03.12 at 10:00, Alex Shi <alex.shi@intel.com> wrote:
> Yes, we can use cast for intermediate data. And actually, current kernel
> has live mis-used case on cmpxchg(), that I plan to point out too.
>
> -- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -203,12 +203,12 @@ static bool make_all_cpus_request(struct kvm *kvm,
> unsigned int req)
>
> void kvm_flush_remote_tlbs(struct kvm *kvm)
> {
> - int dirty_count = kvm->tlbs_dirty;
> + long dirty_count = kvm->tlbs_dirty;
>
> smp_mb();
> if (make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
> ++kvm->stat.remote_tlb_flush;
> - cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);
> + cmpxchg(&kvm->tlbs_dirty, dirty_count, 0L);

Indeed - the cmpxchg would fail if the value doesn't fit. But this is not
to say that in certain cases it isn't valid to pass an int for the second
and/or third argument. (And quite likely the issue here is theoretical
only anyway.)

In particular, requiring an L suffix here on literals should be avoided.

Jan

> }




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