Messages in this thread | | | Date | Wed, 7 Oct 2015 13:01:20 +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 Wed, Oct 07, 2015 at 08:42:05AM +0000, Mathieu Desnoyers wrote: > ----- On Oct 7, 2015, at 3:51 AM, Peter Zijlstra peterz@infradead.org 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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