Re: [RFC PATCH 1/2] lib/percpu-list: Per-cpu list with associated per-cpu locks

[Dave Chinner <david@fromorbit.com>]

On Wed, Feb 17, 2016 at 12:00:40PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 17, 2016 at 08:53:18PM +1100, Dave Chinner wrote:
> > > +/**
> > > + * for_all_percpu_list_entries - iterate over all the per-cpu list with locking
> > > + * @pos:	the type * to use as a loop cursor for the current .
> > > + * @next:	an internal type * variable pointing to the next entry
> > > + * @pchead:	an internal struct list * of percpu list head
> > > + * @pclock:	an internal variable for the current per-cpu spinlock
> > > + * @head:	the head of the per-cpu list
> > > + * @member:	the name of the per-cpu list within the struct
> > > + */
> > > +#define for_all_percpu_list_entries(pos, next, pchead, pclock, head, member)\
> > > +	{								 \
> > > +	int cpu;							 \
> > > +	for_each_possible_cpu (cpu) {					 \
> > > +		typeof(*pos) *next;					 \
> > > +		spinlock_t *pclock = per_cpu_ptr(&(head)->lock, cpu);	 \
> > > +		struct list_head *pchead = &per_cpu_ptr(head, cpu)->list;\
> > > +		spin_lock(pclock);					 \
> > > +		list_for_each_entry_safe(pos, next, pchead, member.list)
> > > +
> > > +#define end_all_percpu_list_entries(pclock)	spin_unlock(pclock); } }
> > 
> > This is a bit of a landmine 
> 
> Yeah, that is pretty terrible. Maybe a visitor interface is advisable?
> 
> visit_percpu_list_entries(struct percpu_list *head, void (*visitor)(struct list_head *pos, void *data), void *data)
> {
> 	int cpu;
> 
> 	for_each_possible_cpu(cpu) {
> 		spinlock_t *lock = per_cpu_ptr(&head->lock, cpu);
> 		struct list_head *head = per_cpu_ptr(&head->list, cpu);
> 		struct list_head *pos, *tmp;
> 
> 		spin_lock(lock);
> 		for (pos = head->next, tmp = pos->next; pos != head; pos = tmp)
> 			visitor(pos, data);

I thought about this - it's the same problem as the list_lru walking
functions. That is, the visitor has to be able to drop the list lock
to do blocking operations, so the lock has to be passed to the
visitor/internal loop context somehow, and the way the callers can
use it need to be documented.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com