Runqueue spinlock recursion on arm64 v4.15

[Peter Zijlstra]

On Mon, Feb 05, 2018 at 01:36:00PM +0000, Mark Rutland wrote:
> On Fri, Feb 02, 2018 at 10:07:26PM +0000, Mark Rutland wrote:
> > On Fri, Feb 02, 2018 at 08:55:06PM +0100, Peter Zijlstra wrote:
> > > On Fri, Feb 02, 2018 at 07:27:04PM +0000, Mark Rutland wrote:
> > > > ... in some cases, owner_cpu is -1, so I guess we're racing with an
> > > > unlock. I only ever see this on the runqueue locks in wake up functions.
> > > 
> > > So runqueue locks are special in that the owner changes over a contex
> > > switch, maybe something goes funny there?
> > 
> > Aha! I think that's it!
> > 
> > In finish_lock_switch() we do:
> > 
> > 	smp_store_release(&prev->on_cpu, 0);
> > 	...
> > 	rq->lock.owner = current;
> > 
> > As soon as we update prev->on_cpu, prev can be scheduled on another CPU, and
> > can thus see a stale value for rq->lock.owner (e.g. if it tries to wake up
> > another task on that rq).
> 
> I hacked in a forced vCPU preemption between the two using a sled of WFE
> instructions, and now I can trigger the problem in seconds rather than
> hours.
> 
> With the patch below applied, things seem to fine so far.
> 
> So I'm pretty sure this is it. I'll clean up the patch text and resend
> that in a bit.

Also try and send it against an up-to-date scheduler tree, we just
moved some stuff around just about there.