[mythtv-users] Could realtime commercial flagging cause hard lockup/freeze/hang?

[Brian Foddy]

On Tuesday 03 June 2008, finlay wrote:
> Greg Grotsky wrote:
> > Hello all,
> >
> >   I've been a mythtv user for a few years.  My configuration is a
> > single system frontend/backend.  Since it's inception I have been
> > using realtime commercial flagging and it has worked very well.
> > However, I recently updated my mythtv svn tree (to 17392 from an
> > update 5/3/08) and every once in a while the system completely hangs.
> > It hangs to the point where the magic Alt-SysRq-REISUB does not reboot
> > it, no SSH, no three-fingered-salute, no nothing.
> >
> >   I get nothing in my logs (FE, BE, syslog, messages, etc) and it
> > appears to happen when recording and doing commercial detection.  It
> > froze three times over three days and I started to worry.  I
> > completely disabled commercial detection and it worked for two days
> > straight so I decided to turn comflagging back on because the queue
> > was huge.  It detected all the commercials in the recorded shows and
> > the next day while recording and comflagging it died again!  So I
> > turned off the realtime comflag option and it has been working for two
> > days now without a hang.
> >
> >   I thought maybe it was a memory issue so I sacrificed a few shows
> > and ran memtest86 overnight; it completed 12 cycles and didn't report
> > any errors.  None of the information I have proves anything but so far
> > it's looking like some update done in may to the comflagger is the
> > culprit.  Anyone have information that supports or debunks this
> > theory?  It's not a big loss to not have realtime comflagging but it
> > was a cool geek thing to tout. :)
>
> I had something similar happen to my system recently while using
> realtime commflag. After much futzing around with hardware I finally
> realized that the OS seemed to be the problem. I had upgraded the system
> from Ubuntu 710 to 804 a few weeks before and everything seemed OK but
> then the problems started - system lockup when recording and
> commflagging but working fine otherwise. I finally installed a clean
> version of Mythbuntu 804 and everything has been working great for the
> last 2 weeks.
> John

I'd guess its some kernel deadlock with a device driver.  What tuner card
is (are) running during the process.  If the machine is really busy, and
you have some of the kernel preempt settings on, and there is a bug
in the driver, it could cause a hard crash.

What type of computer?  Multi-core, dual CPU?  Using SMP kernel?
These can cause driver bugs more readily.  If its an SMP instance,
try your test with SMP disabled (there is a kernel param that will do this).

The commercial scan itself is probably (haven't looked at code, but
I can't imagine) doing anything other than standard file I/O using
normal calls, so root or not, I don't see it the problem.  But if it
keeps the system busy, it could cause other faults to show up.

In general, hard lock ups / crashes are due to faulty / buggy device drivers
or hardware failures.  

Brian