linux-kernel - Re: [PATCH] rbtree: remove unneeded explicit alignment in struct rb

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Date:   Thu, 5 Aug 2021 17:10:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dbueso@...e.de>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Michel Lespinasse <michel@...pinasse.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mete Polat <metepolat2000@...il.com>,
        Jesper Nilsson <jesper@....nu>,
        David Woodhouse <dwmw2@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        kernel-janitors@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] rbtree: remove unneeded explicit alignment in struct
 rb_node

On Thu, Aug 05, 2021 at 08:02:28AM -0700, Davidlohr Bueso wrote:
> On 2021-08-05 07:02, Arnd Bergmann wrote:
> > The revert would appear to change the alignment to 16 bits instead
> > of 32 bits on m68k as well (not 8 bits as on cris), but I don't know if
> > that
> > can cause problems there.
> 
> Yeah I tried this a while back and it broke m68k, so it was a no go:
> 
> https://lore.kernel.org/lkml/CAMuHMdXeZvJ0X6Ah2CpLRoQJm+YhxAWBt-rUpxoyfOLTcHp+0g@mail.gmail.com/

I'm still thinking that any architecture that doesn't respect natural
alignment is playing with fire. For giggles we should put a runtime
alignment check in READ_ONCE() and see what goes *bang*.