Re: [PATCH v2 -tip 0/6] locking: Introduce range reader/writer lock

[Davidlohr Bueso <dave@stgolabs.net>]

On Thu, 20 Apr 2017, Peter Zijlstra wrote:

>For opt spinning we need to specifically know who would be next in
>order, again, doesn't matter how many, just who's next.

I've sent a v3 with a more precise description of this, which I hope is
to your satisfaction.

Given a clear tree iteration/order defined by interval trees (indexed by
lowpoint and treats duplicates as inorder traversal), it is not something
I would wish to alter. Over the weekend I've been experimenting more with
still taking the tree->lock, but spinning while blocking ranges is 1 and
'owner' (in this case the first overlapping node, remembered when we did
the initial lookup adding to the tree, _with_ the tree->lock held) is on_cpu.
This would maintain the order and prevent blocking for threads that are
about (?) to receive the lock.

While I have somewhat of a patch, I'm tired and have not had the chance to
even test the thing, so I went ahead and sent v3 anyway to not delay further.

Thanks,
Davidlohr