Re: [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT

[Davidlohr Bueso]

On Tue, 12 Jan 2016, Peter Zijlstra wrote:

> 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?

I found this because I wanted to use the waiter check in rtmutexes in a lockless
fashion (ie rt_mutex_has_waiters).

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

This too.

Thanks,
Davidlohr