Re: [PATCH 3/4] kvm: powerpc: define a linux pte lookup function

[Paul Mackerras <paulus@samba.org>]

On Tue, Oct 08, 2013 at 11:33:31AM +0530, Bharat Bhushan wrote:
> We need to search linux "pte" to get "pte" attributes for
> setting TLB in KVM.
> This patch defines a linux_pte_lookup() function for same.
> 

[snip]

> +	/* wait until _PAGE_BUSY is clear */
> +	while (1) {
> +		pte = pte_val(*ptep);
> +		if (unlikely(pte & _PAGE_BUSY)) {
> +			cpu_relax();
> +			continue;
> +		}
> +	}
> +
> +	/* If pte is not present return None */
> +	if (unlikely(!(pte & _PAGE_PRESENT)))
> +		return __pte(0);

First, this looks racy to me, since nothing stops the compiler
refetching pte from memory, and it might have become busy again in the
meantime.

However, I don't think you need to do any of this, since the caller in
your next patch checks for pte_present(pte) anyway.  On book E systems
we don't use _PAGE_BUSY, so you don't need the loop for your intended
application.  I suggest you remove the entire section quoted above.

Paul.