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Date: Wed, 15 Jun 2016 20:40:07 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Waiman Long <Waiman.Long@hpe.com>, Ingo Molnar <mingo@redhat.com>,
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        Jason Low <jason.low2@hp.com>, Dave Chinner <david@fromorbit.com>,
        Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>
Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper
 acquire/release barrier
Message-ID: <20160615184007.GW30921@twins.programming.kicks-ass.net>
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On Wed, Jun 15, 2016 at 11:27:24AM -0700, Davidlohr Bueso wrote:
> On Wed, 15 Jun 2016, Peter Zijlstra wrote:
> 
> >In any case, its fairly simple to cure, just add
> >smp_acquire__after_ctrl_dep() at the end. If we bail because
> >need_resched() we don't need the acquire I think.
> 
> I was just considering this for your smp_cond_acquire/smp_cond_load_acquire

Right, so that need_resched break makes that a bit awkward. Not to
mention the cpu_relaxed() vs cpu_relaxed_lowlatency() difference.

> rework, so yeah I guess an smp_acquire__after_ctrl_dep would be a nice
> compromise.
> 
> However, I was always under the impression that races with node->locked were
> rather harmless (as indicated in the mentioned commit) -- which is why ->locked
> are simple load/stores, with the exception of the unqueueing -- but yeah, that's
> not even paired.

Yeah, see a few patches further in this series, where he guards a
variables with the osq_lock.