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

[Davidlohr Bueso]

On Thu, 20 Apr 2017, Peter Zijlstra wrote:

> On Thu, Apr 20, 2017 at 10:13:26AM -0700, Davidlohr Bueso wrote:
>
> > I have thought of some heuristics for avoiding sleeping under certain
> > constraints, which could mitigate the spinning step we loose, but I fear it
> > will never be exactly as fast as rwsems -- just consider
> > we always take the tree->lock.
>
> But tree->lock is a spinlock, so while this gets us out of rwsem-xadd
> territory for the fast paths, the whole lock-stealing and optimistic
> spinning stuff is on a different scale.

Oh, absolutely. I was merely pointing out the differences at a hair
splitting level.

> Those are about avoiding actually going to sleep and having to be woken
> up (and waiting to become running) again, which is a long time.

Yes, which is why I was thinking of ways to mitigate this. Ie: for
blocked writers with low counts of 'blocking_ranges'.

Thanks,
Davidlohr