Re: NULL pointer dereference in pick_next_task_fair

[Qais Yousef]

On 11/06/19 14:08, Peter Zijlstra wrote:
> On Wed, Nov 06, 2019 at 01:05:25PM +0100, Peter Zijlstra wrote:
> > On Mon, Oct 28, 2019 at 05:46:03PM +0000, Quentin Perret wrote:
> > > 
> > > After digging a bit, the offending commit seems to be:
> > > 
> > >     67692435c411 ("sched: Rework pick_next_task() slow-path")
> > > 
> > > By 'offending' I mean that reverting it makes the issue go away. The
> > > issue comes from the fact that pick_next_entity() returns a NULL se in
> > > the 'simple' path of pick_next_task_fair(), which causes obvious
> > > problems in the subsequent call to set_next_entity().
> > > 
> > > I'll dig more, but if anybody understands the issue in the meatime feel
> > > free to send me a patch to try out :)
> > 
> > So for all those who didn't follow along on IRC, the below seems to cure
> > things.
> > 
> > The only thing I'm now considering is if we shouldn't be setting
> > ->on_cpu=2 _before_ calling put_prev_task(). I'll go audit the RT/DL
> > cases.
> 
> So I think it all works, but that's more by accident than anything else.
> I'll move the ->on_cpu=2 assignment earlier. That clearly avoids calling
> put_prev_task() while we're in put_prev_task().

Did you mean avoids calling *set_next_task()* while we're in put_prev_task()?

So what you're saying is that put_prev_task_{rt,dl}() could drop the rq_lock()
too and the race could happen while we're inside these functions, correct? Or
is it a different reason?

By the way, is all reads/writes to ->on_cpu happen when a lock is held? Ie: we
don't need to use any smp read/write barriers?

Cheers

--
Qais Yousef