Re: Re: NULL pointer dereference in pick_next_task_fair

[Peter Zijlstra]

On Wed, Nov 06, 2019 at 05:54:37PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 06, 2019 at 06:51:40PM +0300, Kirill Tkhai wrote:
> > > +#ifdef CONFIG_SMP
> > > +	if (!rq->nr_running) {
> > > +		/*
> > > +		 * Make sure task_on_rq_curr() fails, such that we don't do
> > > +		 * put_prev_task() + set_next_task() on this task again.
> > > +		 */
> > > +		prev->on_cpu = 2;
> > >  		newidle_balance(rq, rf);
> > 
> > Shouldn't we restore prev->on_cpu = 1 after newidle_balance()? Can't prev
> > become pickable again after newidle_balance() releases rq->lock, and we
> > take it again, so this on_cpu == 2 never will be cleared?
> 
> Indeed so.

Oh wait, the way it was written this is not possible. Because
rq->nr_running == 0 and prev->on_cpu > 0 it means the current task is
going to sleep and cannot be woken back up.

But if I move the ->on_cpu=2 thing earlier, as I wrote I'd do, then yes,
we have to set it back to 1. Because in that case we can get here for a
spurious schedule and we'll pick the same task again.