Re: [PATCH 06/10] time: Cap clocksource reads to the clocksource max_cycles value

[John Stultz]

On Tue, Jan 13, 2015 at 3:11 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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
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