Re: [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms

["Andy Lutomirski" <luto@kernel.org>]

On Thu, Jun 17, 2021, at 2:13 AM, Peter Zijlstra wrote:
> On Thu, Jun 17, 2021 at 11:08:03AM +0200, Peter Zijlstra wrote:
> > +static void mmdrop_lazy(struct rq *rq)
> > +{
> > +	struct mm_struct *old_mm;
> > +
> > +	if (likely(!READ_ONCE(rq->drop_mm)))
> > +		return;
> > +
> > +	/*
> > +	 * Slow path.  This only happens when we recently stopped using
> > +	 * an mm that is exiting.
> > +	 */
> > +	old_mm = xchg(&rq->drop_mm, NULL);
> > +	if (old_mm)
> > +		mmdrop(old_mm);
> > +}
> 
> AFAICT if we observe a !NULL value on the load, the xchg() *MUST* also
> see !NULL (although it might see a different !NULL value). So do we want
> to write it something like so instead?

Like so?

> 
> static void mmdrop_lazy(struct rq *rq)
> {
> 	struct mm_struct *old_mm;
> 
> 	if (likely(!READ_ONCE(rq->drop_mm)))
> 		return;
> 
> 	/*
> 	 * Slow path.  This only happens when we recently stopped using
> 	 * an mm that is exiting.

* This xchg is the only thing that can change rq->drop_mm from non-NULL to NULL, and
* multiple mmdrop_lazy() calls can't run concurrently on the same CPU.

> 	 */
> 	old_mm = xchg(&rq->drop_mm, NULL);
> 	if (WARN_ON_ONCE(!old_mm))
> 		return;
> 
> 	mmdrop(old_mm);
> }
> 

--Andy