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

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Message-ID: <20151020195529.423f0dbb@gandalf.local.home>
Date:	Tue, 20 Oct 2015 19:55:29 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] tracing: Have stack tracer force RCU to be
 watching

On Tue, 20 Oct 2015 16:48:30 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> 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.
> 
> Please!  ;-) ;-) ;-)
> 
>

That's a separate fix, as it will break elsewhere than just this.

As for my patch, Can I have an Acked-by?

Thanks!

-- Steve
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