perf events ring buffer memory barrier on powerpc

[Peter Zijlstra]

On Tue, Oct 29, 2013 at 11:30:57AM +0100, Peter Zijlstra wrote:
> @@ -154,9 +175,11 @@ int perf_output_begin(struct perf_output
>  		 * Userspace could choose to issue a mb() before updating the
>  		 * tail pointer. So that all reads will be completed before the
>  		 * write is issued.
> +		 *
> +		 * See perf_output_put_handle().
>  		 */
>  		tail = ACCESS_ONCE(rb->user_page->data_tail);
> -		smp_rmb();
> +		smp_mb();
>  		offset = head = local_read(&rb->head);
>  		head += size;
>  		if (unlikely(!perf_output_space(rb, tail, offset, head)))

That said; it would be very nice to be able to remove this barrier. This
is in every event write path :/