Messages in this thread | | | Date | Wed, 7 Oct 2015 09:51:14 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to allow consolidation |
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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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