Subject: Re: [PATCH 1/3] sfq: timer is deferrable

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 18 Jan 2008 20:34:46 -0800

On Fri, Jan 18, 2008 at 02:49:00PM -0800, Stephen Hemminger wrote:
> The perturbation timer used for re-keying can be deferred, it doesn't
> need to be deterministic.

The only concern that I can come up with is that the sfq_perturbation
timer might be on one CPU, and all the operations using the corresponding
SFQ on another.  This could in theory allow a nearly omniscient attacker
to exploit an SFQ imbalance while preventing perturbation of the hash
function.

This does not seem to be a valid concern at this point, since there are
very few uses of init_timer_deferrable().  And if it should become a
problem, one approach would be to have some sort of per-timer limit to
the deferral.  Of course, at that point one would need to figure out
what this limit should be!

Acked-by: Paul E. McKenney <[email-masked]>

> Signed-off-by: Stephen Hemminger <[email-masked]>
> 
> 
> --- a/net/sched/sch_sfq.c	2008-01-17 08:29:24.000000000 -0800
> +++ b/net/sched/sch_sfq.c	2008-01-17 09:00:58.000000000 -0800
> @@ -426,7 +426,9 @@ static int sfq_init(struct Qdisc *sch, s
>  	struct sfq_sched_data *q = qdisc_priv(sch);
>  	int i;
> 
> -	setup_timer(&q->perturb_timer, sfq_perturbation, (unsigned long)sch);
> +	q->perturb_timer.function = sfq_perturbation;
> +	q->perturb_timer.data = (unsigned long)sch;;
> +	init_timer_deferrable(&q->perturb_timer);
> 
>  	for (i=0; i<SFQ_HASH_DIVISOR; i++)
>  		q->ht[i] = SFQ_DEPTH;