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

[Mete Polat]

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.