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Re: [libvirt] [PATCH] util: make it more robust to calculate timeout value



On Fri, May 15, 2015 at 01:09:09PM +0200, Michal Privoznik wrote:
> On 15.05.2015 08:26, zhang bo wrote:
> > When we change system clock to years ago, a certain CPU may use up 100% cputime.
> > The reason is that in function virEventPollCalculateTimeout(), we assign the 
> > unsigned long long result to an INT variable, 
> >         *timeout = then - now; // timeout is INT, and then/now are long long
> >         if (*timeout < 0)
> >             *timeout = 0;
> > there's a chance that variable @then minus variable @now may be a very large number 
> > that overflows INT value expression, then *timeout will be negative and be assigned to 0.
> > Next the 'poll' in function virEventPollRunOnce() will get into an 'endless' while loop there.
> > thus, the cpu that virEventPollRunOnce() thread runs on will go up to 100%. 
> > 
> > Although as we discussed before in https://www.redhat.com/archives/libvir-list/2015-May/msg00400.html
> > it should be prohibited to set-time while other applications are running, but it does 
> > seems to have no harm to make the codes more robust.
> > 
> > Signed-off-by: Wang Yufei <james wangyufei huawei com>
> > Signed-off-by: Zhang Bo <oscar zhangbo huawei com>
> > ---
> >  src/util/vireventpoll.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/src/util/vireventpoll.c b/src/util/vireventpoll.c
> > index ffda206..5f5a149 100644
> > --- a/src/util/vireventpoll.c
> > +++ b/src/util/vireventpoll.c
> > @@ -357,9 +357,10 @@ static int virEventPollCalculateTimeout(int *timeout)
> >              return -1;
> >  
> >          EVENT_DEBUG("Schedule timeout then=%llu now=%llu", then, now);
> > -        *timeout = then - now;
> > -        if (*timeout < 0)
> > +        if (then < now)
> >              *timeout = 0;
> > +        else
> > +            *timeout = (then - now) & 0x7FFFFFFF;
> 
> You're trying to make this an unsigned value. What's wrong with plain
> typecast?
> 
> >      } else {
> >          *timeout = -1;
> >      }
> > 
> 
> I must say this is ugly. If the system clock is changed, all the
> timeouts should fire, shouldn't they? Otherwise important events can be
> missed.

As I said in previous thread, I think that this is really just papering
over one specific issue, and you are still going to have a multitude of
problems with every app on the system when you change the system clock
in this kind of way. I'm just not convinced we should be trying to hack
around it in this one case, as it is just giving us a false illusion
that things are going to continue to work, when in reality they'll just
break somewhere else instead. eg the pthread_cond_wait() timeouts.


Regards,
Daniel
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