Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup

From: Jeff Merkey
Date: Wed Jan 20 2016 - 12:36:12 EST

On 1/20/16, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> On Wed, Jan 20, 2016 at 9:16 AM, Jeff Merkey <linux.mdb@xxxxxxxxx> wrote:
>> On 1/20/16, Jeff Merkey <linux.mdb@xxxxxxxxx> wrote:
>>> On 1/20/16, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>>> 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()
>> I guess I am going to have to become an expert on the timekeeper and
>> learn this subsystem backwards and forwards to code a touch function
>> to keep it from crashing the system.
>> On the 2.6 series kernels (and 2.2) this problem did not exist. I
>> noticed a lot of these changes came in in the late 2.6 cycles. Before
>> that time, I could leave the debugger spinning for days and linux
>> worked fine.
>> For people who have to pay developers to develop code on Linux a
>> debugger is almost
>> an essential tool since it saves hundreds of thousands of dollars in
>> development costs. Not everyone wants to spend money for their
>> employees and engineers to sit around and code review every problem -
>> customers just want their problems fixed -- and fast. That being
>> said, I am having no lack of people who download and use this debugger
>> and I'm certain kgdb is heavily used by folks doing development. If
>> kernel development is too hard, people move to something else based on
>> simple economics.
>> That being said, I need to get this fixed. There is no good reason a
>> debugger shouldn't be able to stop the system and leave it suspended
>> for days if necessary to run down a bug. I wrote a debugger on SMP
>> Netware that worked that way. The earliest versions of MDB worked
>> that way.
>> kgdb is broken right now because of this. I am not certain it affects
>> all systems out there, but it needs to be fixed.
>> If you have any ideas on how to code a touch function please send me a
>> patch or suggest how it could be done non-obstrusively, otherwise I'll
>> have to dive into the timekeeper and fix it myself and learn yet
>> another subsystem of Linux and fix it bugs. A code subsystem that
>> crashes because the timer tick is skewed or returns garbage is poorly
>> designed IMHO.
> Ehrm. A more productive route in solving this might be to cap the
> cycle delta we return from timekeeping_get_delta().
> We already do this in the CONFIG_DEBUG_TIMEKEEPING, but adding a
> simple check it to the non-debug case should be doable w/o adding too
> much overhead to this very hot path.
> Something like:
> if (delta > tkr->clock->max_cycles)
> delta = tkr->clock->max_cycles;
> return delta;
> thanks
> -john

Thank you John. This is helpful. Can you send me a patch for this
and I'll test it. Then I am not touching this code and you guys can
put it in.