Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier
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- To: Davidlohr Bueso <dave@xxxxxxxxxxxx>
- Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier
- From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
- Date: Wed, 15 Jun 2016 20:40:07 +0200
- Cc: Waiman Long <Waiman.Long@xxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx, linux-alpha@xxxxxxxxxxxxxxx, linux-ia64@xxxxxxxxxxxxxxx, linux-s390@xxxxxxxxxxxxxxx, linux-arch@xxxxxxxxxxxxxxx, Jason Low <jason.low2@xxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Scott J Norton <scott.norton@xxxxxxx>, Douglas Hatch <doug.hatch@xxxxxxx>
- In-reply-to: <20160615182724.GD2094@linux-80c1.suse>
- List-id: <linux-ia64.vger.kernel.org>
- References: <1465944489-43440-1-git-send-email-Waiman.Long@hpe.com> <1465944489-43440-2-git-send-email-Waiman.Long@hpe.com> <20160615165659.GC2094@linux-80c1.suse> <20160615171250.GO30921@twins.programming.kicks-ass.net> <20160615182724.GD2094@linux-80c1.suse>
- User-agent: Mutt/1.5.23.1 (2014-03-12)
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.
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