linux-kernel - Re: [PATCH 0/8] linker-section array fix and clean ups

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Date:   Fri, 6 Nov 2020 18:02:27 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Johan Hovold <johan@...nel.org>, Jessica Yu <jeyu@...nel.org>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nick Desaulniers <ndesaulniers@...ogle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        David Miller <davem@...emloft.net>,
        Jakub Jelinek <jakub@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Kurtz <djkurtz@...omium.org>,
        linux-arch@...r.kernel.org, linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH 0/8] linker-section array fix and clean ups

On Fri, Nov 06, 2020 at 11:55:23AM -0500, Steven Rostedt wrote:
> On Fri, 6 Nov 2020 17:45:37 +0100
> Johan Hovold <johan@...nel.org> wrote:
> 
> > It's simply specifying alignment when declaring the variable that
> > prevents this optimisation. The relevant code is in the function
> > align_variable() in [1] where DATA_ALIGNMENT() is never called in case
> > an alignment has been specified (!DECL_USER_ALIGN(decl)).
> > 
> > There's no mention in the documentation of this that I'm aware of, but
> > this is the way the aligned attribute has worked since its introduction
> > judging from the commit history.
> > 
> > As mentioned above, we've been relying on this for kernel parameters and
> > other structures since 2003-2004 so if it ever were to change we'd find
> > out soon enough.
> > 
> > It's about to be used for scheduler classes as well. [2]
> 
> Is this something that gcc folks are aware of? Yes, we appear to be relying
> on undocumented implementations, but that hasn't caused gcc to break the
> kernel in the past.

The scheduler change was suggested by Jakub so at least some of them
are.

Johan