perf events ring buffer memory barrier on powerpc

[Peter Zijlstra]

On Wed, Oct 30, 2013 at 07:29:30PM +0100, Peter Zijlstra wrote:
> +	page_shift = PAGE_SHIFT + page_order(rb);
> +
> +	handle->page = (offset >> page_shift) & (rb->nr_pages - 1);
> +
> +	offset &= page_shift - 1;

offset &= (1UL << page_shift) - 1;

Weird that it even appeared to work.. /me wonders if he even booted the
right kernel.

> +
> +	handle->addr = rb->data_pages[handle->page] + offset;
> +	handle->size = (1 << page_shift) - offset;