Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to allow consolidation

[Peter Zijlstra]

On Tue, Oct 06, 2015 at 01:58:50PM -0700, Paul E. McKenney wrote:
> On Tue, Oct 06, 2015 at 10:29:37PM +0200, Peter Zijlstra wrote:
> > On Tue, Oct 06, 2015 at 09:29:21AM -0700, Paul E. McKenney wrote:
> > > +static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
> > > +					      struct rcu_node *rnp, bool wake)
> > > +{
> > > +	unsigned long flags;
> > > +	unsigned long mask;
> > > +
> > > +	raw_spin_lock_irqsave(&rnp->lock, flags);
> > 
> > Normally we require a comment with barriers, explaining the order and
> > the pairing etc.. :-)
> > 
> > > +	smp_mb__after_unlock_lock();
> 
> Hmmmm...  That is not good.
> 
> Worse yet, I am missing comments on most of the pre-existing barriers
> of this form.

Yes I noticed.. :/

> The purpose is to enforce the heavy-weight grace-period memory-ordering
> guarantees documented in the synchronize_sched() header comment and
> elsewhere.

> They pair with anything you might use to check for violation
> of these guarantees, or, simiarly, any ordering that you might use when
> relying on these guarantees.

I'm sure you know what that means, but I've no clue ;-) That is, I
wouldn't know where to start looking in the RCU implementation to verify
the barrier is either needed or sufficient. Unless you mean _everywhere_
:-)

> I could add something like  "/* Enforce GP memory ordering. */"
> 
> Or perhaps "/* See synchronize_sched() header. */"
> 
> I do not propose reproducing the synchronize_sched() header on each
> of these.  That would be verbose, even for me!  ;-)
> 
> Other thoughts?

Well, this is an UNLOCK+LOCK on non-matching lock variables upgrade to
full barrier thing, right?

To me its not clear which UNLOCK we even match here. I've just read the
sync_sched() header, but that doesn't help me either, so referring to
that isn't really helpful either.

In any case, I don't want to make too big a fuzz here, but I just
stumbled over a lot of unannotated barriers and figured I ought to say
something about it.
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