Re: [PATCH v2] rbtree: remove unneeded explicit alignment in struct rb_node

[Peter Zijlstra <peterz@infradead.org>]

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).