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

[Peter Zijlstra]

On Wed, Oct 07, 2015 at 08:42:05AM +0000, Mathieu Desnoyers wrote:
> ----- On Oct 7, 2015, at 3:51 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:
> 
> > 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_
> > :-)
> 
> One example is the new membarrier system call. It relies on synchronize_sched()
> to enforce this:

That again doesn't explain which UNLOCKs with non-matching lock values
it pairs with and what particular ordering is important here.

I'm fully well aware of what sync_sched() guarantees and how one can use
it, that is not the issue, what I'm saying is that a generic description
of sync_sched() doesn't help in figuring out WTH that barrier is for and
which other code I should also inspect.
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