On Tue, Jan 13, 2015 at 01:33:29PM -0800, John Stultz wrote: > On Tue, Jan 13, 2015 at 3:11 AM, Peter Zijlstra <pet...@infradead.org> wrote: > > On Fri, Jan 09, 2015 at 04:34:24PM -0800, John Stultz wrote: > >> When calculating the current delta since the last tick, we > >> currently have no hard protections to prevent a multiplciation > >> overflow from ocurring. > >> > >> This patch introduces such a cap that limits the read delta > >> value to the max_cycles value, which is where an overflow would > >> occur. > > > >> +++ b/kernel/time/timekeeping.c > >> @@ -202,6 +202,9 @@ static inline s64 timekeeping_get_ns(struct > >> tk_read_base *tkr) > >> /* calculate the delta since the last update_wall_time: */ > >> delta = clocksource_delta(cycle_now, tkr->cycle_last, tkr->mask); > >> > >> + /* Cap delta value to the max_cycles values to avoid mult overflows > >> */ > >> + delta = min(delta, tkr->clock->max_cycles); > >> + > >> nsec = delta * tkr->mult + tkr->xtime_nsec; > >> nsec >>= tkr->shift; > >> > > > > So while I appreciate stuff can be broken, should we not at least keep > > track of this brokenness? That is, we all agree bad things happened IF > > we actually hit this, right? So should we then not inform people that > > bad things did happen? > > So since this is a time reading function, this could be called > anywhere. So I'm hesitant to try to printk anything in such a hot > path. Though, if we catch such a large delta during the timekeeping > update function, we will print a warning (which is done in an earlier > patch in the series). > > Were you thinking of something else maybe? I guess we could set a flag > and then print later (if there is a later), but we'd lose much of the > context of what went wrong.
Maybe a stats counter? In any case, keeping it silent seems the wrong thing. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
