linux-kernel - Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier

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Message-ID: <20160615185611.GE2094@linux-80c1.suse>
Date:	Wed, 15 Jun 2016 11:56:11 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Waiman Long <Waiman.Long@....com>, Ingo Molnar <mingo@...hat.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-arch@...r.kernel.org,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper
 acquire/release barrier

On Wed, 15 Jun 2016, Peter Zijlstra wrote:

>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.

Oh sure, I was merely refering to the ordering semantics, not the calls
themselves -- although at some point, as archs begin to port locking/core
optimizations, we _will_ need the variants for dealing with '_lowlatency'.

>
>> 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.

*sigh*