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

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Message-ID: <alpine.LFD.0.98.0705230752040.3890@woody.linux-foundation.org>
Date:	Wed, 23 May 2007 08:00:38 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Patrick McHardy <kaber@...sh.net>
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

On Wed, 23 May 2007, Patrick McHardy wrote:
> 
> Yes, that looks better, thanks.

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.

Hmm? What _should_ that thing do?

		Linus
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