linux-kernel - Re: [PATCH v2] rbtree: remove unneeded explicit alignment in struct rb

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Message-ID: <20210806174457.GA2184@worktop.programming.kicks-ass.net>
Date:   Fri, 6 Aug 2021 19:44:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mete Polat <metepolat2000@...il.com>
Cc:     Michel Lespinasse <michel@...pinasse.org>,
        Davidlohr Bueso <dbueso@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Jesper Nilsson <jesper@....nu>, Arnd Bergmann <arnd@...db.de>,
        David Woodhouse <dwmw2@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        kernel-janitors@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH v2] rbtree: remove unneeded explicit alignment in struct
 rb_node

On Fri, Aug 06, 2021 at 05:22:08PM +0200, Mete Polat wrote:
> Commit e977145aeaad ("[RBTREE] Add explicit alignment to sizeof(long) for
> struct rb_node.") adds an explicit alignment to the struct rb_node due to
> some speciality of the CRIS architecture.
> 
> The support for the CRIS architecture was removed with commit c690eddc2f3b
> ("CRIS: Drop support for the CRIS port")
> 
> So, remove this now unneeded explicit alignment in struct rb_node as well.
> 
> This basically reverts commit e977145aeaad ("[RBTREE] Add explicit
> alignment to sizeof(long) for struct rb_node.").
> 
> The rbtree node color is stored in the LSB of '__rb_parent_color'.
> Only mask the first bit in '__rb_parent()', otherwise it modifies the
> node's parent address on m68k.

I still don't believe for a second this will actually work. We rely on
rcu_assign_pointer() and rcu_dereference() to work on the
rb_{left,right} members, and I don't think any architecture can provide
single copy atomic loads and stores that are not naturally aligned (eg.
when they straddle a cache or page boundary).