Re: RFC: PERCPU_LIST to fix PR kern/52515

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

> Date: Fri, 8 Dec 2017 18:51:28 +0900
> From: Kengo NAKAHARA <k-nakahara%iij.ad.jp@localhost>
> 
> However, it seems your patch has large diff... From the point of code
> stability, smaller diff SLIST version would be better for netbsd-8 branch
> to fix the bug. Because your patch causes some new ATF failures such as
> ldp_regen and route_change_ifp (reported by ozaki-r@n.o). We can probably
> fix them at once but guaranteeing its stability would take more time.

Not surprising I made a bug somewhere in there!  The SLIST version is
fine by me.  However, there's one small nit:

> diff --git a/sys/sys/psref.h b/sys/sys/psref.h
> index 88db6dbb603..9096a3798d6 100644
> --- a/sys/sys/psref.h
> +++ b/sys/sys/psref.h
> @@ -69,7 +69,7 @@ struct psref_target {
>   *	written only on the local CPU.
>   */
>  struct psref {
> -	LIST_ENTRY(psref)		psref_entry;
> +	SLIST_ENTRY(psref)		psref_entry;
>  	const struct psref_target	*psref_target;

In order to avoid changing the kernel ABI in netbsd-8, please add a
void *psref_unused0 after psref_entry.  That way we don't have to
think about the consequences of an ABI change in the branch, and we
still have the opportunity to switch back to a doubly-linked list if
we want.