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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 7 May 2007 09:28:32 -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 Mon, 7 May 2007, Esben Nielsen wrote:
> 
> What is (long)(a-b) ? I have tried to look it up in the C99 standeard but I
> can't find it. Maybe it is in the referred LIA-1 standeard, which I can't find
> with google.

I don't worry about non-2's-complement machines (they don't exist, and 
likely won't exist in the future either).

So I worry about compilers rewriting my code.

So "(long)(a-b) < 0" (with "a" and "b" being unsigned long) is basically a 
portable way of testing the high bit of the result.

> I think the best would be to use "a-b > ULONG_MAX/2" when you mean "a<b" as
> that should be completely portable.

That certainly works too, but the difference is irrelevant, since Linux is 
unlikely to work on insane machines anyway (ie we do make a lot of other 
assumptions about the architecture, being two's-complement is the least of 
those).

So you basically shouldn't worry about hardware: everybody is pretty much 
the same. You should worry about *compilers* - that's where the 
differences show up.

So "(long)(a-b)" may be "implementation defined" (but since 
implementations are all 2's complement, we don't care), but a signed 
"(a-b)" that over/overflows is *undefined*, and that is much worse because 
it means that the compiler can do some funky stuff, and _that_ is a real 
practical worry.

And no, I also don't worry about porting Linux to 18-bit machines, or to 
ternary CPU's.  

			Linus
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/