linux-kernel - Re: [PATCH 1/2] Use faster check for modules in backtrace on 64bit

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Message-ID: <20140930085814.GS5430@worktop>
Date:	Tue, 30 Sep 2014 10:58:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andi Kleen <ak@...ux.intel.com>, dave@...1.net,
	linux-kernel@...r.kernel.org, mingo@...nel.org, eranian@...gle.com,
	x86@...nel.org
Subject: Re: [PATCH 1/2] Use faster check for modules in backtrace on 64bit

On Mon, Sep 29, 2014 at 10:30:23PM +0200, Andi Kleen wrote:
> On Mon, Sep 29, 2014 at 08:21:45AM -0700, Andi Kleen wrote:
> > On Mon, Sep 29, 2014 at 01:42:12PM +0200, Peter Zijlstra wrote:
> > > On Fri, Sep 26, 2014 at 04:31:16PM -0700, Andi Kleen wrote:
> > > > 
> > > > This has the (small) potential to get a false positive on a pointer to a
> > > > data segment in a module.  However since we also use the frame pointer
> > > > chain as initial sanity check I think the danger of this is very low.
> > > > 
> > > 
> > > So this has come up several times; and the answer has always been, why
> > > not make the __module_address() thing a rb-tree instead of a linear
> > > loop. So I suppose I'll ask that again, why not?
> > 
> > Why do things complicated, if they can be done simple too?
> 
> Also I investigated it now, but we don't have RCU support for rbtrees.
> So it would need some kind of locking for the reader, which is a show
> stopper.

Nah, we can trivially do that with a seqlock. Not read side locking
required in the normal case.
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