netdev - Re: bad networking related lag in v2.6.22-rc2

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Date:	Wed, 23 May 2007 19:16:30 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Herbert Xu <herbert@...dor.apana.org.au>,
	Ingo Molnar <mingo@...e.hu>,
	Anant Nitya <kernel@...chanda.info>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: bad networking related lag in v2.6.22-rc2

Linus Torvalds wrote:
> There appear to be other obvious problems in the recent "cleanups" in this 
> area..
> 
> Look at
> 
> 	psched_tdiff_bounded(psched_time_t tv1, psched_time_t tv2, psched_time_t bound)
> 	{
> 		return min(tv1 - tv2, bound);
> 	}
> 
> and compare it to the previous code:
> 
> 	#define PSCHED_TDIFF_SAFE(tv1, tv2, bound) \
> 		min_t(long long, (tv1) - (tv2), bound)
> 
> and ponder how that "trivial cleanup" totally broke the thing. 
> 
> Hint: "psched_time_t" is an "u64". What does that mean for
> 
> 	min(tv1 - tv2, bound);
> 
> again, when "tv2" is larger than tv1. It _used_ to return a negative 
> value. Now it returns a positive "bound" upper bound, because "tv1-tv2" 
> will be used as a huge unsigned (and thus _positive_) integer. And was 
> that accidental, or done on purpose?
> 
> Sounds accidental to me, since you then want to return a "psched_tdiff_t", 
> which is typedeffed to be "long".
>
> Doesn't sound very safe to me, especially since the commit message for 
> this is "[NET_SCHED]: turn PSCHED_TDIFF_SAFE into inline function", and 
> there's no indication that anybody realized that it changed semantics in 
> the process.


I did realize it, but tv2 > tv1 can't happen and makes no sense for
the users of this function. I probably should have provided a more
detailed changelog entry.

> Hmm? What _should_ that thing do?


It is used to calculate the amount of tokens a tocken bucket has
accumulated since the last refill, thus we always have tv1 >= tv2
(modulo ktime wraps). In fact tv2 > tv1 was never properly
supported. This macro would have returned the negative long long
value, but all users assign it to a psched_tdiff_t (long), so
depending on the exact values, it might still be interpreted as a
large positive value. Additionally there was a second implementation
for the gettimeofday clocksource that didn't return the negative
difference but the bound value.

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