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

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Message-ID: <20090422171047.GA5975@nowhere>
Date:	Wed, 22 Apr 2009 19:10:51 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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, 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.



> Thanks,
> 
> -- Steve
> 

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