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Date: Wed, 20 Nov 2002 16:15:33 -0500
From: Luben Tuikov <luben@splentec.com>
Organization: Splentec Ltd.
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To: Alan Cox <alan@lxorguk.ukuu.org.uk>
CC: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, axboe@suse.de
Subject: Re: [PATCH]: jiffies wrap in ll_rw_blk.c
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Alan Cox wrote:
> 
> On Wed, 2002-11-20 at 21:00, Luben Tuikov wrote:sk) {
> > > > -               unsigned long duration = jiffies - req->start_time;
> > > > +               unsigned long duration = (signed) jiffies - (signed) req->start_time;
> > > >                 switch (rq_data_dir(req)) {
> > >
> > > It was right before. Your patch breaks it. Think about it in unsigned
> > > maths
> > >
> > >               0x00000002 - 0xFFFFFFFF = 0x00000003
> >
> > 0x2 - (-0x1) = 0x2 + 0x1 = 0x3
> >
> > Right! I thought (signed) does the job. I actually tried
> > it both ways and works all right. I guess either way works fine.
> 
> (signed long) maybe - but not signed - long is 64bit on Alpha, (signed)
> is 32

Aaaah, I see where you're coming from.

I basically tried to stay away from knowing _what_ actual
type it is and just to make it signed and do the arithmetic,
but didn't know the specific for the Alpha.

Shouldn't this be (symbolically:) (signed (typeof(jiffes)) jiffies -
(signed (typeof(start)) start -- you know what I mean.

But yes both ways should work:
a, b are both unsigned, then a-b = a+(-b) and there's just no
other way to compute the result.

Anyway, doesn't matter,
-- 
Luben
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