Re: [PATCH tip/locking/core v9 6/6] locking/pvqspinlock: Queue node adaptive spinning

[Peter Zijlstra]

On Fri, Nov 06, 2015 at 12:54:06PM -0500, Waiman Long wrote:
> >>+static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
> >>  {
> >>  	struct pv_node *pn = (struct pv_node *)node;
> >>+	struct pv_node *pp = (struct pv_node *)prev;
> >>  	int waitcnt = 0;
> >>  	int loop;
> >>+	bool wait_early;
> >>
> >>  	/* waitcnt processing will be compiled out if !QUEUED_LOCK_STAT */
> >>  	for (;; waitcnt++) {
> >>-		for (loop = SPIN_THRESHOLD; loop; loop--) {
> >>+		for (wait_early = false, loop = SPIN_THRESHOLD; loop; loop--) {
> >>  			if (READ_ONCE(node->locked))
> >>  				return;
> >>+			if (pv_wait_early(pp, loop)) {
> >>+				wait_early = true;
> >>+				break;
> >>+			}
> >>  			cpu_relax();
> >>  		}
> >>
> >So if prev points to another node, it will never see vcpu_running. Was
> >that fully intended?
> 
> I had added code in pv_wait_head_or_lock to set the state appropriately for
> the queue head vCPU.

Yes, but that's the head, for nodes we'll always have halted or hashed.
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