Messages in this thread | | | Date | Mon, 9 Oct 2017 16:37:44 +0200 | From | Andrea Parri <> | Subject | Re: [PATCH tip/core/rcu 1/9] rcu: Provide GP ordering in face of migrations and delays |
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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..
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