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

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Date:   Tue, 10 Aug 2021 21:15:39 +0200
From:   Mete Polat <metepolat2000@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 07:44:57PM +0200, Peter Zijlstra wrote:
> 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).
> 

I guess I am misunderstanding something here, but isn't that then a
problem that all rcu pointers in any struct would face, independent of
an 'aligned' struct attribute? As long as allocators do not place a
small struct as rb_node over page boundaries and the rcu pointers itself
are aligned we should be fine, aren't we? I am not sure if any of the
SL*B allocators is doing that though.