Messages in this thread | | | Date | Thu, 17 Jun 2021 11:13:56 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms |
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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?
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 (WARN_ON_ONCE(!old_mm)) return;
mmdrop(old_mm); }
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