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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.
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