Re: 2.6.14-rc4-rt1 - enable IRQ-off tracing causes kernel to fault at boot

From: Mark Knecht
Date: Fri Oct 14 2005 - 10:06:35 EST

On 10/13/05, Ingo Molnar <mingo@xxxxxxx> wrote:
> * Mark Knecht <markknecht@xxxxxxxxx> wrote:
> > Ingo & Steve,
> > Thank you for your great instructions that even a guitar player
> > could basically follow. After about an hour of messing around I did
> > manage to capture the crash. The console file is attached.
> >
> > NOTE: The first time I booted the kernel it got to the crash point and
> > the machine rebooted. The second time it booted I got the trace. Both
> > boots are in the capture file.
> thanks, this log is much more informative. No smoking gun though, but it
> seems something fundamental (probably lowlevel x64 code) has been broken
> by -rt1.
> Do the crashes go away if you take the -rc3-rt13 version of
> arch/x86_64/kernel/entry.S and copy it over into the -rc4-rt1 tree?
> [this undoes a particular set of CONFIG_CRITICAL_IRQSOFF_TIMING fixes
> from the x64 code, which i did during -rc3-rt13 => -rc4-rt1]

Indeed it is fixed by doing this. Options are on but the modified
kernel does boot:

* *
* REMINDER, the following debugging options are turned on in your .config: *
* *
* *
* they may increase runtime overhead and latencies. *
* *

mark@lightning ~ $ uname -a
Linux lightning 2.6.14-rc4-rt1 #8 PREEMPT Fri Oct 14 07:46:29 PDT 2005
x86_64 AMD Athlon(tm) 64 Processor 3000+ AuthenticAMD GNU/Linux
mark@lightning ~ $

> (Note that doing this will re-introduce tracing bugs, which can result
> in false-positive latency readings - but it should fix any related
> lowlevel bug in the assembly code.)
> if this indeed solves the crash then i'd suggest to restore the -rt1
> version of entry.S, and i'd suggest to disable CRITICAL_IRQSOFF_TIMING
> until i fix it. You should be able to get pretty good latency tracing
> info even without CRITICAL_IRQSOFF_TIMING.
> Ingo

Will got back to the original entry.S file with the IRQoff option
turned off. Let me know when you have a fix to test, or if you need
more data.

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