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

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Message-ID: <20201106115523.41f7e2ed@gandalf.local.home>
Date:   Fri, 6 Nov 2020 11:55:23 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     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, 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.

-- Steve