Re: [PATCH 1/3] 2.6.14-rc2-mm1: fixes for overflow msec_to_jiffies()

[Andrew Morton <akpm@osdl.org>]

Willy Tarreau <willy@w.ods.org> wrote:
>
> Thanks Andrew,
> 
> I'm very sorry because I have verified the code with gcc-2.95.3,
> gcc-3.3.6 and gcc-3.4.4 on x86 and alpha to ensure that everything
> went smooth on archs where sizeof(long) > sizeof(int). But I've
> tested all the combinations in user-space for obvious ease of
> validation. I believe I forgot to use -Wall. What architecture
> gave you this, and with which compiler please ? I'm willing to
> fix this as soon as I can understand the root of the problem.
> 

http://www.zip.com.au/~akpm/linux/patches/stuff/top-posting.txt

> 
> 
> On Thu, Sep 29, 2005 at 02:43:12AM -0700, Andrew Morton wrote:
> > Willy Tarreau <willy@w.ods.org> wrote:
> > >
> > > +#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
> > >  +#  define MAX_MSEC_OFFSET \
> > >  +	(ULONG_MAX - (MSEC_PER_SEC / HZ) + 1)
> > 
> > That generates numbers which don't fit into unsigned ints, yielding vast
> > numbers of
> > 
> > include/linux/jiffies.h: In function `msecs_to_jiffies':
> > include/linux/jiffies.h:310: warning: comparison is always false due to limited range of data type
> > include/linux/jiffies.h: In function `usecs_to_jiffies':
> > include/linux/jiffies.h:323: warning: comparison is always false due to limited range of data type
> > 

This was a ppc64 build, gcc-3.3.3, CONFIG_HZ=250

Look a the value which MAX_MSEC_OFFSET will take (it's 2^63 minus a bit). 
Comparing that to an unsigned int will generate the always-true or
always-false warning.