Re: [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT

From: Peter Zijlstra
Date: Tue Jan 12 2016 - 04:06:55 EST

On Mon, Jan 11, 2016 at 10:22:43AM -0800, Davidlohr Bueso wrote:
> With d72da4a4d97 (rbtree: Make lockless searches non-fatal) our rbtrees
> provide weak guarantees that allows us to do lockless (and very speculative)
> reads of the tree. Such readers cannot see partial stores on nodes, ie
> left/right as well as root. As such, similar to the WRITE_ONCE semantics when
> doing rotations, use READ_ONCE when checking the root node in RB_EMPTY_ROOT.

No objection, but is this actually used anywhere?

Or is this a just-in-case completeness thing?