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

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Message-ID: <20080119055700.GC24840@linux.vnet.ibm.com>
Date:	Fri, 18 Jan 2008 21:57:00 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Stephen Hemminger <stephen.hemminger@...tta.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/3] sfq: timer is deferrable

On Fri, Jan 18, 2008 at 08:36:55PM -0800, Stephen Hemminger wrote:
> On Fri, 18 Jan 2008 20:34:46 -0800
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > 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 <paulmck@...ux.vnet.ibm.com>
> 
> But the only threat is getting more bandwidth for a longer interval.
> It is all kind of moot anyway because the bandwidth hogs all open
> multiple connections anyway, so SFQ is of no use.

Good point, and an additional reason for my Acked-by above.  ;-)

						Thanx, Paul
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