linux-kernel - Re: [PATCH 11/12] jump_label: declare jump table as external array

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Message-ID: <20161017173309.7020e084@gandalf.local.home>
Date:   Mon, 17 Oct 2016 17:33:09 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Vegard Nossum <vegard.nossum@...cle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Jason Baron <jbaron@...hat.com>
Subject: Re: [PATCH 11/12] jump_label: declare jump table as external array

On Sun, 16 Oct 2016 18:50:55 +0200
Vegard Nossum <vegard.nossum@...cle.com> wrote:
> >
> > NAK, -ENOCHANGELOG.

Agreed.

> >  
> 
> Hi Peter,
> 
> It's true I didn't put an RFC tag on this (mostly because git-send-email
> doesn't seem to have an option for it?), but the whole point of doing

I would think it does, although I never use it (I always use quilt
mail).


> these other patches (03-12) was to demonstrate what the patches would
> look like for some other kernel code and ask for feedback on the overall
> interface/approach. I don't know if you read the introduction and first
> patch in the series, but I'd expect that to be more than enough to
> understand the problem.

But we were not Cc'd on those. If we are not on the Cc to the
introduction nor the other patches, we will most likely not be reading
them.

> 
> If we really have to repeat the rationale for every patch, can we reuse
> this?
> 
> "Comparisons between pointers to different arrays is technically
> undefined behaviour and recent GCCs may incorrectly optimise away loop
> termination conditions. Use the external array accessor macros to
> prevent this from happening."
> 

So basically gcc will break on these array address calculations? Which
version of gcc started this, and has this actually been an issue?


-- Steve