Re: [PATCH 0/2] [GIT PULL] tracing: various bug fixes

[Jeremy Fitzhardinge]

Frederic Weisbecker wrote:
> On Wed, Apr 22, 2009 at 09:49:14AM -0400, Steven Rostedt wrote:
>   
> > On Wed, 22 Apr 2009, Frederic Weisbecker wrote:
> >     
> > > > I spent the entire day (and half the night) debugging this. I was fighting 
> > > > a case where the hardirqs_enabled flag in the task struct (lockdep flag) 
> > > > was mysteriously being set and cleared. I stepped through the entire 
> > > > kernel thread fork process (that was an exercise) and could not find 
> > > > anything wrong.
> > > >
> > > > Sometimes it would go away with printk's sometimes it would not. This was 
> > > > driving me crazy, until I noticed that paravirt was enabled.
> > > >
> > > > Turning off paravirtualization here (so far) makes everything run 
> > > > smoothly.
> > > >
> > > > Thus my theory is that there's something fishy with the modifying of the 
> > > > irq enable/disable code when the system detects that it is running on bare 
> > > > hardware.
> > > >
> > > > I'm too tired to look at this more. Ingo supplied a config to play with. 
> > > > You can disable VSMP too and it will still trigger the crash.
> > > >
> > > > -- Steve
> > > >
> > > >         
> > > It's indeed a tricky one. I can reproduce it too, I will
> > > try to manage having an irqsoff trace at this point, hopefully I
> > > could get the source of this irq disabling...
> > >       
> > It doesn't disable interrupts :-/
> >
> > It is the hardirqs_enabled flag in the task struct that mysteriously turns 
> > off and back on. I put in printks when it is off in fork, and the next 
> > printk shows that it turns back on (between the printks!!!).
> >
> > I printed the output of "irqs_disabled()" on each of these printks and 
> > interrupts are always enabled. It is only the hardirqs_enabled flag that 
> > is giving strange outputs.
> >     
>
>
> Oh, weird...
>
>  
>   
> > Do you have CONFIG_PARAVIRT on?  When I disabled it, I have yet to 
> > reproduce the bug. But I've only rebooted a few times. I'm going to 
> > continue to reboot to see if I can trigger it.
> >     
>
>
> Yes it is enabled.
>
>
>
>   
> > I'm thinking that the paravirt alternative code may have clobbered a 
> > register in either the enable or disabling of interrupts. This might cause 
> > a strange value to go into the hardirqs_enabled flag.
> >     
>
>
>
> Ok I will try it without PARAVIRT and tell you if I can reproduce it.
>   

Interesting.  What code is generated for native_irq_enable/disable?

   J
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