Re: [PATCH] mm/mmu_notifier: rename mmu_notifier_synchronize() to <...>_barrier()

[Sean Christopherson]

On Mon, Nov 05, 2018 at 12:18:33PM -0800, Andrew Morton wrote:
> On Mon,  5 Nov 2018 11:29:55 -0800 Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> 
> > ...and update its comment to explicitly reference its association with
> > mmu_notifier_call_srcu().
> > 
> > Contrary to its name, mmu_notifier_synchronize() does not synchronize
> > the notifier's SRCU instance, but rather waits for RCU callbacks to
> > finished, i.e. it invokes rcu_barrier().  The RCU documentation is
> > quite clear on this matter, explicitly calling out that rcu_barrier()
> > does not imply synchronize_rcu().  The misnomer could lean an unwary
> > developer to incorrectly assume that mmu_notifier_synchronize() can
> > be used in conjunction with mmu_notifier_unregister_no_release() to
> > implement a variation of mmu_notifier_unregister() that synchronizes
> > SRCU without invoking ->release.  A Documentation-allergic and hasty
> > developer could be further confused by the fact that rcu_barrier() is
> > indeed a pass-through to synchronize_rcu()... in tiny SRCU.
> 
> Fair enough.
> 
> > --- a/mm/mmu_notifier.c
> > +++ b/mm/mmu_notifier.c
> > @@ -35,12 +35,12 @@ void mmu_notifier_call_srcu(struct rcu_head *rcu,
> >  }
> >  EXPORT_SYMBOL_GPL(mmu_notifier_call_srcu);
> >  
> > -void mmu_notifier_synchronize(void)
> > +void mmu_notifier_barrier(void)
> >  {
> > -	/* Wait for any running method to finish. */
> > +	/* Wait for any running RCU callbacks (see above) to finish. */
> >  	srcu_barrier(&srcu);
> >  }
> > -EXPORT_SYMBOL_GPL(mmu_notifier_synchronize);
> > +EXPORT_SYMBOL_GPL(mmu_notifier_barrier);
> >  
> >  /*
> >   * This function can't run concurrently against mmu_notifier_register
> 
> But as it has no callers, why retain it?

I was hesitant to remove it altogether since it was explicitly added to
complement mmu_notifier_call_srcu()[1] even though the initial user of
mmu_notifier_call_srcu() didn't use mmu_notifier_synchronize()[2].  I
assume there was a good reason for adding the barrier function, but
maybe that's a bad assumption.

[1] b972216e27d1 ("mmu_notifier: add call_srcu and sync function for listener to delay call and sync")
[2] https://lore.kernel.org/patchwork/patch/515318/