Re: [PATCH tip/core/rcu 1/9] rcu: Provide GP ordering in face of migrations and delays

[Andrea Parri]

On Mon, Oct 09, 2017 at 10:16:37AM +0200, Peter Zijlstra wrote:
> On Sat, Oct 07, 2017 at 11:28:57AM -0700, Paul E. McKenney wrote:
> > But if you are saying that it would be good to have wait_for_completion()
> > and complete() directly modeled at some point, no argument.  In addition,
> > I hope that the memory model is applied to other tools that analyze kernel
> > code.
> 
> > > I'm not sure I got the point across; so I'll try once more. Without
> > > providing this ordering the completion would be fundamentally broken. It
> > > _must_ provide this ordering.
> > 
> > OK, I now understand what you are getting at, and I do very much like
> > that guarantee.
> 
> Right, so maybe we should update the completion comments a bit to call
> out this property, because I'm not sure its there.
> 
> Also, with this, I think the smp_store_release() / smp_load_acquire() is
> a perfectly fine abstraction of it, I don't think the model needs to be
> taught about the completion interface.
> 
> > > Why not? In what sort of cases does it go wobbly?
> > 
> > For one, when it conflicts with maintainability.  For example, it would
> > probably be OK for some of RCU's rcu_node ->lock acquisitions to skip the
> > smp_mb__after_unlock_lock() invocations.  But those are slowpaths, and the
> > small speedup on only one architecture is just not worth the added pain.
> > Especially given the nice wrapper functions that you provided.
> > 
> > But of course if this were instead (say) rcu_read_lock() or common-case
> > rcu_read_unlock(), I would be willing to undergo much more pain.  On the
> > other hand, for that exact reason, that common-case code path doesn't
> > acquire locks in the first place.  ;-)
> 
> Ah, so for models I would go absolutely minimal; it helps understand
> what the strict requirements are and where we over-provide etc..

Except, maybe, that simplicity and maintainability do apply to "models"
(design) as well... ;-)

As Ingo once put it [1] (referring to the "Linux-kernel memory model"):

  "it's not true that Linux has to offer a barrier and locking model
   that panders to the weakest (and craziest!) memory ordering model
   amongst all the possible Linux platforms - theoretical or real metal.

   Instead what we want to do is to consciously, intelligently _pick_
   a sane, maintainable memory model and offer primitives for that -
   at least as far as generic code is concerned. Each architecture can
   map those primitives to the best of its abilities."

  Andrea

[1] https://marc.info/?l=linux-mm&m=138513336717990&w=2


> 
> For actual code you're entirely right, there's no point in trying to be
> cute with the rcu-node locks. Simplicity rules etc..