Re: Runqueue spinlock recursion on arm64 v4.15

[Mark Rutland]

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 guess the below (completely untested) would fix that. I'll try to give it a
go next week.

Thanks,
Mark.

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