netdev - Re: [PATCH 09/10] Change perturb

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Message-ID: <4700FA02.8090800@trash.net>
Date:	Mon, 01 Oct 2007 15:45:38 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Corey Hickey <bugfood-ml@...ooh.org>
CC:	netdev@...r.kernel.org
Subject: Re: [PATCH 09/10] Change perturb_period to unsigned.

Corey Hickey wrote:
> perturb_period is currently a signed integer, but I can't see any good
> reason why this is so--a negative perturbation period will add a timer
> that expires in the past, causing constant perturbation, which makes
> hashing useless.
> 
> 	if (q->perturb_period) {
> 		q->perturb_timer.expires = jiffies + q->perturb_period;
> 		add_timer(&q->perturb_timer);
> 	}
> 
> Strictly speaking, this will break binary compatibility with older
> versions of tc, but that ought not to be a problem because (a) there's
> no valid use for a negative perturb_period, and (b) negative values
> will be seen as high values (> INT_MAX), which don't work anyway.
> 
> If perturb_period is too large, (perturb_period * HZ) will overflow the
> size of an unsigned int and wrap around. So, check for thet and reject
> values that are too high.


Sounds reasonable.

> --- a/net/sched/sch_sfq.c
> +++ b/net/sched/sch_sfq.c
> @@ -74,6 +74,9 @@
>  typedef unsigned int sfq_index;
>  #define SFQ_MAX_DEPTH (UINT_MAX / 2 - 1)
>  
> +/* We don't want perturb_period * HZ to overflow an unsigned int. */
> +#define SFQ_MAX_PERTURB (UINT_MAX / HZ)


jiffies are unsigned long.
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