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
