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

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Date:   Mon, 9 Oct 2017 10:16:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com
Subject: Re: [PATCH tip/core/rcu 1/9] rcu: Provide GP ordering in face of
 migrations and delays

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..

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