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. thanks -john -- 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/
