Re: [PATCH] Priority Lists for the RT mutex

[Oleg Nesterov <oleg@tv-sign.ru>]

Oleg Nesterov wrote:
>
> Daniel Walker wrote:
> >
> > Description:
> > 	This patch adds the priority list data structure from Inaky Perez-Gonzalez
> > to the Preempt Real-Time mutex.
> >
> ...
>
> I can't understand how this can work.

And I think it is possible to simplify plist's design.

struct plist {
	int prio;
	struct list_head prio_list;
	struct list_head node_list;
};

#define	plist_for_each(pos, head)	\
	 list_for_each_entry(pos, &(head)->node_list, node_list)

static inline void plist_add(struct plist *new, struct plist *head)
{
	struct plist* pos;

	INIT_LIST_HEAD(&new->prio_list);

	list_for_each_entry(pos, &head->prio_list, prio_list)
		if (new->prio < pos->prio)
			goto lt_prio;
		else if (new->prio == pos->prio) {
			pos = list_entry(pos->prio_list.next,
					 struct plist, prio_list);
			goto eq_prio;
		}

lt_prio:
	list_add_tail(&new->prio_list, &pos->prio_list);
eq_prio:
	list_add_tail(&new->node_list, &pos->node_list);
}

static inline void plist_del(struct plist *del)
{
	if (!list_empty(&del->prio_list)) {
		struct plist *next = list_entry(del->node_list.next,
						struct plist, node_list);
		list_move_tail(&next->prio_list, &del->prio_list);
		list_del_init(&del->prio_list);
	}

	list_del_init(&del->node_list);
}

All nodes in prio list are chained via ->node_list, sorted in ascending
->prio/fifo order. Nodes which start different priority slice are chained
via ->prio_list, ascending ->prio order.

Slightly tested, seems to work.

I think this would be even better:

struct plist_head {
	struct list_head prio_list;
	struct list_head node_list;
}

struct plist {
	int prio;
	struct plist_head head;
}

The plist_head's min/max prio can be found from
plist_head.prio_list.next/prev.

What do you think?

Oleg.