Re: [PATCH v3 2/3] locking/mutex: Enable optimistic spinning of woken task in wait queue

[Peter Zijlstra]

On Tue, Mar 29, 2016 at 05:39:35PM +0200, Peter Zijlstra wrote:
> On Tue, Mar 22, 2016 at 01:46:43PM -0400, Waiman Long wrote:
> > Ding Tianhong reported a live-lock situation where a constant stream
> > of incoming optimistic spinners blocked a task in the wait list from
> > getting the mutex.
> > 
> > This patch attempts to fix this live-lock condition by enabling the
> > woken task in the wait queue to enter into an optimistic spinning
> > loop itself in parallel with the regular spinners in the OSQ. This
> > should prevent the live-lock condition from happening.
> 
> I would very much like a few words on how fairness is preserved.
> 
> Because while the waiter remains on the wait_list while it spins, and
> therefore unlock()s will only wake it, and we'll only contend with the
> one waiter, the fact that we have two spinners is not fair or starvation
> proof at all.

Alternatively, we can say this is good enough until proven deficient,
but then we should still very much document this.