linux-kernel - Re: [RFC][PATCH] tracing: Have stack tracer force RCU to be watching

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Message-ID: <20151021080142.GZ2881@worktop.programming.kicks-ass.net>
Date:	Wed, 21 Oct 2015 10:01:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] tracing: Have stack tracer force RCU to be watching

On Tue, Oct 20, 2015 at 05:32:28PM -0400, Steven Rostedt wrote:
> On Tue, 20 Oct 2015 13:25:28 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Tue, Oct 20, 2015 at 12:10:31PM -0400, Steven Rostedt wrote:
> > > 
> > > Paul,
> > > 
> > > I've spent a couple of days debugging this, and finally found that my
> > > stack tracer was calling the stack trace code, which calls
> > > __module_address() which asserts the below.
> > > 
> > > Is just calling rcu_irq_enter() and rcu_irq_exit() safe to do
> > > everywhere (with interrupts always disabled)? This patch appears to fix
> > > the bug.
> > 
> > Yep!  Just don't call it from an NMI handler.  And don't call it with
> > interrupts enabled.  The patch looks to have interrupts always disabled,
> > and the surrounding code doesn't look like NMI-safe code anyway, so
> > should be OK.
> > 
> > 							Thanx, Paul
> >
> 
> Hmm, good point about NMI handler. Right now I think the only thing
> protecting this from getting in the critical section while in NMI is
> the check that we are using the task struct stack. But that may not be
> enough in 32 bit.
> 
> I should probably add a "if (in_nmi()) return" somewhere.

But if there's an arch that doesn't use a separate NMI stack, the NMI
might cause the largest stack, which would then remain invisible from
the stack-tracer.

Should we not instead fix the NMI-safety of this tracer?
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