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

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Date:	Wed, 21 Oct 2015 13:52:38 +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 Wed, Oct 21, 2015 at 07:39:22AM -0400, Steven Rostedt wrote:
> On Wed, 21 Oct 2015 10:01:42 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > > 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?
> 
> We could, but that should be a separate project, as that would require
> doing everything lockless, which would require a redesign. Is that
> worth it?

I've no idea on either, not on how hard it would be to fix, nor on if
its worth the effort.

I suppose auditing which archs do not have dedicated NMI stacks might be
a good first stab at things. x86 still uses an IST for NMIs, right?
Andy's been changing things a lot lately, I'm not sure I'm up to date on
this.

> For now, the safe thing to do is the if (in_nmi()), but certainly, if
> someone gets time to make it NMI safe, we can do that too.

Sure, fix current holes first.
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