Re: [patch 2/7] CAN: Add PF_CAN core module

Paul E. McKenney
2007-05-18T14:34:01Z

On Fri, May 18, 2007 at 11:19:01AM +0200, Oliver Hartkopp wrote:
> Hi Urs, Hello Paul,
>
> i assume Paul refers to the can_rx_delete_all() function that adds each
> receive list entry for rcu removal using the can_rx_delete RCU callback,
> right?
>
> So the idea would be to create a second RCU callback - e.g.
> can_rx_delete_list() - that removes the complete list inside the RCU
> callback?!?
> The list removal would therefore be processed inside this new
> can_rx_delete_list() in RCU context and not inside can_rx_delete_all().
>
> @Paul: Was this your intention?

My intention was that the list-removing be placed into can_rcv_lists_delete(),
perhaps as follows:

static void can_rx_delete_all(struct hlist_head *rl)
{
struct receiver *r;
struct hlist_node *n;

hlist_for_each_entry(r, n, rl, list) {
hlist_del(&r->list);
kmem_cache_free(rcv_cache, r);
}
}

static void can_rcv_lists_delete(struct rcu_head *rp)
{
struct dev_rcv_lists *d = container_of(rp, struct dev_rcv_lists, rcu);

/* remove all receivers hooked at this netdevice */
can_rx_delete_all(&d->rx_err);
can_rx_delete_all(&d->rx_all);
can_rx_delete_all(&d->rx_fil);
can_rx_delete_all(&d->rx_inv);
can_rx_delete_all(&d->rx_eff);
for (i = 0; i < 2048; i++)
can_rx_delete_all(&d->rx_sff[i]);
kfree(d);
}

Then the code in can_notifier() can reduce to the following:

if (d) {
hlist_del_rcu(&d->list);

/* used to be a string of can_rx_delete_all(). */
} else
printk(KERN_ERR "can: notifier: receive list not "
"found for dev %s\n", dev->name);

spin_lock_bh(&rcv_lists_lock);

if (d) {
call_rcu(&d->rcu, can_rcv_lists_delete);
}

This moves the traversal work into the callback function. This is not
a problem for CONFIG_PREEMPT_RT and non-CONFIG_PREEMPT, but not sure
about CONFIG_PREEMPT.

But it sure has the potential to cut down on a bunch of call_rcu()
work...

Thanx, Paul

> Best regards,
> Oliver
>
> Paul E. McKenney wrote:
> >On Wed, May 16, 2007 at 04:51:02PM +0200, Urs Thuermann wrote:
> >
> >>This patch adds the CAN core functionality but no protocols or drivers.
> >>No protocol implementations are included here. They come as separate
> >>patches. Protocol numbers are already in include/linux/can.h.
> >>
> >
> >Interesting! One question called out below -- why do call_rcu() on each
> >piece of the struct dev_rcv_lists, instead of doing call_rcu() on the
> >whole thing and having the RCU callback free up the pieces? Given that
> >all the pieces are call_rcu()ed separately, there had better not be
> >persistent pointers to the pieces, right?
> >
> >Doing it in one chunk would make the code a bit simpler and also reduce
> >the RCU overhead a bit.
> >
> >Or am I missing something subtle here?
> >
> > Thanx, Paul