Re: [Patch] Re: Nasty suprise with uptime

[Tim Schmielau]

On Thu, 1 Nov 2001, george anzinger wrote:

> Tim Schmielau wrote:
> > +
> > +#if BITS_PER_LONG < 48
> > +
> > +u64 get_jiffies64(void)
> > +{
> > +       static unsigned long jiffies_hi = 0;
> > +       static unsigned long jiffies_last = INITIAL_JIFFIES;
> > +       unsigned long jiffies_tmp;
> > +
> > +       jiffies_tmp = jiffies;   /* avoid races */
> > +       if (jiffies_tmp < jiffies_last)   /* We have a wrap */
> > +               jiffies_hi++;
> > +       jiffies_last = jiffies_tmp;
> > +
> > +       return (jiffies_tmp | ((u64)jiffies_hi) << BITS_PER_LONG);
>
> Doesn't this need to be protected on SMP machines?  What if two cpus
> call get_jiffies64() at the same time...  Seems like jiffies_hi could
> get bumped twice instead of once.
>
> George
>

Yes, it does, my race protection is bogus. Petr Vandrovec also pointed out
that. So we do need either to
 a) stuff jiffies_hi and jiffies_last into one atomic type
    (16 bits is enough for each) or
 b) use locking.
My next patch will use b), but I won't do it until I have resolved the
most annoying stability issues. I won't have time to do this before the
weekend, and don't want to bother the list too much either.

Maybe the lockups are just due to my setting of INITIAL_JIFFIES instead of
waiting 471 days. The time adjustment routines are good candidates for
this kind of mistakes. Any ideas anyone where else I might have forgotten
to introduce INITIAL_JIFFIES ?

Tim