linux-kernel - Re: [RFC PATCH 10/10] x86/unwind: add undwarf unwinder

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Date:   Thu, 1 Jun 2017 15:12:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>, Jiri Slaby <jslaby@...e.cz>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCH 10/10] x86/unwind: add undwarf unwinder

On Thu, Jun 01, 2017 at 07:36:09AM -0500, Josh Poimboeuf wrote:
> On Thu, Jun 01, 2017 at 02:13:56PM +0200, Peter Zijlstra wrote:
> > So we do that lookup for every single frame. That's going to hurt.
> > 
> > Would it make sense to cache the last 'module' in an attempt to at least
> > avoid that lookup again? Something like so:
> 
> The only thing with caching the module is, what if the module goes away?

Yeah.. *boom* ;-) We could of course play games with module_get() and
module_put(), but meh.

> Based on your previous comment I was thinking I would disable preemption
> for the entire unwind_next_frame() step, but not *between* steps.  I
> suppose we could require the unwind caller to disable preemption but I'd
> like to avoid that if possible.

Right, keeping it disabled across a frame should be ok I suppose.