linux-kernel - Re: [patch] CFS scheduler, -v8

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Date:	Sat, 5 May 2007 10:44:28 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Esben Nielsen <nielsen.esben@...glemail.com>
cc:	Ingo Molnar <mingo@...e.hu>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>,
	Gene Heskett <gene.heskett@...il.com>, Mark Lord <lkml@....ca>,
	Zach Carter <linux@...hcarter.com>,
	buddabrod <buddabrod@...il.com>
Subject: Re: [patch] CFS scheduler, -v8



On Sat, 5 May 2007, Esben Nielsen wrote:
> 
> I have been wondering why you use usigned for timers anyway. It is also like
> that in hrtimers. Why not use signed and avoid (almost) all worries about wrap
> around issues. The trick is that when all
>   a < b
> is be replaced by
>   a - b < 0
> the code will work on all 2-complement machines even if the (signed!) integers
> a and b wrap around.

No. BOTH of the above are buggy.

The C language definition doesn't allow signed integers to wrap (ie it's 
undefined behaviour), so "a-b < 0" can be rewritten by the compiler as a 
simple signed "a < b".

And the unsigned (or signed) "a < b" is just broken wrt any kind of 
wrap-around (whether wrapping around zero or the sign bit).

So the _only_ valid way to handle timers is to
 - either not allow wrapping at all (in which case "unsigned" is better, 
   since it is bigger)
 - or use wrapping explicitly, and use unsigned arithmetic (which is 
   well-defined in C) and do something like "(long)(a-b) > 0".

Notice? The signed variant is basically _never_ correct.

		Linus
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