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

[Andy Lutomirski]

On Thu, Jun 17, 2021, at 2:28 AM, Peter Zijlstra wrote:
> On Thu, Jun 17, 2021 at 11:08:03AM +0200, Peter Zijlstra wrote:
> 
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index e595e77913eb..57415cca088c 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -1104,6 +1104,8 @@ static inline void __mmput(struct mm_struct *mm)
> >  	}
> >  	if (mm->binfmt)
> >  		module_put(mm->binfmt->module);
> > +
> > +	mm_unlazy_mm_count(mm);
> >  	mmdrop(mm);
> >  }
> >  
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 8ac693d542f6..e102ec53c2f6 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -19,6 +19,7 @@
> 
> > +/*
> > + * This converts all lazy_mm references to mm to mm_count refcounts.  Our
> > + * caller holds an mm_count reference, so we don't need to worry about mm
> > + * being freed out from under us.
> > + */
> > +void mm_unlazy_mm_count(struct mm_struct *mm)
> > +{
> > +	unsigned int drop_count = num_possible_cpus();
> > +	int cpu;
> > +
> > +	/*
> > +	 * mm_users is zero, so no cpu will set its rq->lazy_mm to mm.
> > +	 */
> > +	WARN_ON_ONCE(atomic_read(&mm->mm_users) != 0);
> > +
> > +	/* Grab enough references for the rest of this function. */
> > +	atomic_add(drop_count, &mm->mm_count);
> 
> So that had me puzzled for a little while. Would something like this be
> a better comment?
> 
> 	/*
> 	 * Because this can race with mmdrop_lazy(), mm_count must be
> 	 * incremented before setting any rq->drop_mm value, otherwise
> 	 * it is possible to free mm early.
> 	 */

Nope, because the caller already did it.  It's an optimization, but maybe it's a poorly done optimization -- I'd rather do two atomic ops than many.

How about:

drop_count = 0;

...

if (!drop_count) {
   /* Collect lots of references.  We'll drop the ones we don't use. */
  drop_count = num_possible_cpus();
  atomic_inc(drop_count, &->mm_count);
}
drop_count--;