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

From: Waiman Long
Date: Thu Mar 31 2016 - 16:40:46 EST

On 03/29/2016 12:42 PM, Peter Zijlstra wrote:
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.

Yes, we can certainly do that.