On Fri, 2 Sep 2016, Thomas Gleixner wrote:
> On Thu, 1 Sep 2016, Liav Rehana wrote:
> > From: Liav Rehana <li...@mellanox.com>
> >
> > During the calculation of the nsec variable, "delta * tkr->mult" may cause
> > overflow to the msb, if the suspended time is too long.
> > In that case, we need to guarantee that the variable will not go through a
> > sign extension during its shift, and thus it will result in a much higher
> > value - close to the larget value of 64 bits.
> > The following commit fixes this problem, which causes the following bug:
> > Trying to connect through ftp to the os after a long enough suspended time
> > will cause the nsec variable to get a much higher value after its shift
> > because of sign extension, and thus the loop that follows some instructions
> > afterwards, implemented in the inline function __iter_div_u64_rem, will
> > take too long.
> >
> > Signed-off-by: Liav Rehana <li...@mellanox.com>
> > ---
> > kernel/time/timekeeping.c | 2 +-
> > 1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> > index 479d25c..ddf56a5 100644
> > --- a/kernel/time/timekeeping.c
> > +++ b/kernel/time/timekeeping.c
> > @@ -305,7 +305,7 @@ static inline s64 timekeeping_delta_to_ns(struct
> > tk_read_base *tkr,
> > s64 nsec;
> >
> > nsec = delta * tkr->mult + tkr->xtime_nsec;
> > - nsec >>= tkr->shift;
> > + nsec = ((u64) nsec) >> tkr->shift;
>
> This typecast is just a baindaid. What happens if you double the suspend time?
> The multiplication will simply overflow. So the proper fix is to sanity check
> delta and do multiple conversions if delta is big enough. Preferrably this
> happens somewhere at the call site and not in this hotpath function.
As a side note. John, why is that stuff unsigned at all? Shouldn't we use
u64 for all of this?
Thanks,
tglx
