On Thu, Aug 05, 2021 at 08:02:28AM -0700, Davidlohr Bueso wrote: > On 2021-08-05 07:02, Arnd Bergmann wrote: > > The revert would appear to change the alignment to 16 bits instead > > of 32 bits on m68k as well (not 8 bits as on cris), but I don't know if > > that > > can cause problems there. > > Yeah I tried this a while back and it broke m68k, so it was a no go: > > https://lore.kernel.org/lkml/CAMuHMdXeZvJ0X6Ah2CpLRoQJm+YhxAWBt-rUpxoyfOLTcHp+0g@xxxxxxxxxxxxxx/ The problem is that the field '__rb_parent_color' in struct rb_node is storing the color AND the pointer to the parent node at the same time. The color is stored in the least significant bit which is fine when rb_node is at least 16-bit aligned. I guess, it does not work on m68k because the makro #define __rb_parent(pc) ((struct rb_node *)(pc & ~3)) used to retrieve the parent pointer zeros the first two bits, not only the first one. Maybe the effiency to store this one color bit in another field was required in the early days but I think moving the color to a seperate field is really the better way to go. It also makes reasoning about the algorithm easier. I will create a patch. Mete > > Thanks, Davidlohr
