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

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Date:	Wed, 22 Apr 2009 09:49:14 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 0/2] [GIT PULL] tracing: various bug fixes




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.

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.

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.

Thanks,

-- Steve

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