linux-kernel - Re: [git pull] timer fix

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Message-ID: <alpine.LFD.2.00.0902041412420.3247@localhost.localdomain>
Date:	Wed, 4 Feb 2009 14:16:23 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [git pull] timer fix

On Wed, 4 Feb 2009, Linus Torvalds wrote:
> 
> Either cast the result of the subtract to "s32" (or "int", whatever), or 
> cast _both_ of them to (s32) so that the subtract is done in a signed 
> type, and then the expansion to (long) will still be right - but 
> unnecessary - in the sign.

Btw, doing it with a nice helper macro or function is also perhaps a good 
idea, at least if these "compare hpet values" things happen more than 
once. 

Look at "time_after()" in <linux/jiffies.h> to see how to do these kinds 
of "comparisons of things that may overflow" really carefully. You 
absolutely need to do the compare in a size that is no larger than the 
size of the actual values (and in the case of HPET, it's 32-bit, at least 
the way we do things now - I guess HPET's _could_ be 64-bit, but we don't 
read more than 32 bits or whatever).

So <linux/jiffies.h> does the cast to "(long)", but it does so because the 
incoming values really have type "unsigned long" and are valid in all 
bits.

			Linus
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