Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup

From: Thomas Gleixner
Date: Wed Jan 20 2016 - 04:23:01 EST

On Tue, 19 Jan 2016, Jeff Merkey wrote:
> Nasty bug but trivial fix for this. What happens here is RAX (nsecs)
> gets set to a huge value (RAX = 0x17AE7F57C671EA7D) and passed through

And how exactly does that happen?

0x17AE7F57C671EA7D = 1.70644e+18 nsec
= 1.70644e+09 sec
= 2.84407e+07 min
= 474011 hrs
= 19750.5 days
= 54.1109 years

That's the real issue, not what you are trying to 'fix' in timespec_add_ns()

> Submitting a patch to fix this after I regress and test it. Since it
> makes no sense to loop on a simple calculation, fix should be:
> static __always_inline void timespec_add_ns(struct timespec *a, u64 ns)
> {
> a->tv_sec += div64_u64_rem(a->tv_nsec + ns, NSEC_PER_SEC, &ns);
> a->tv_nsec = ns;
> }

No. It's not that simple, because div64_u64_rem() is expensive on 32bit
architectures which have no hardware 64/32 division. And that's going to hurt
for the normal tick case where we have at max one iteration.