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SubjectRE: Serious bug in recent Linux kernels
Date
hello,
I'm not too sure about the use of this bit of code,
but if the problem is overflow (i.e. timeout*HZ > 31 bits) then
the problem could be fixed by throwing away a couple of bits
of resolution (which doesn't really happen in practice) by using
the following line.

timeout = (timeout*(HZ/4)+249)/250 + 1;

cheers
Greg Smart

> -----Original Message-----
> From: Linus Torvalds [SMTP:torvalds@transmeta.com]
> Sent: Monday, January 11, 1999 5:33 PM
> To: Alan Modra
> Cc: Guest section DW; Jes.Sorensen@cern.ch;
> linux-kernel@vger.rutgers.edu; jongk@cs.utwente.nl
> Subject: Re: Serious bug in recent Linux kernels
>
>
>
> On Mon, 11 Jan 1999, Alan Modra wrote:
> >
> > There is still a problem for values of timeout a little larger than
> > ((unsigned long)-1 / HZ). eg. for HZ==100, a timeout of 42949673 gets
> > turned into a timeout of 2.
> >
> > better is something like
> >
> > if ((unsigned long)timeout > (unsigned long)-999 / HZ)
> > timeout = MAX_SCHEDULE_TIMEOUT;
> > else if (timeout)
> > timeout = (timeout*HZ+999)/1000+1;
>
> Heh. Even this is incorrect, but it's _really_ close to being right.
>
> The problem is that you're doing a signed division with 1000 if I'm not
> mistaken, and thus even if "timeout*HZ+999" doesn't overflow in 32 bits,
> it's enough that it oevrflows in 31..
>
> Making the 1000 be "1000UL" should fix it. But I'm too lazy to test.
>
> Linus
>
>
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