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

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Message-ID: <20160615182724.GD2094@linux-80c1.suse>
Date:	Wed, 15 Jun 2016 11:27:24 -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:

>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
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.

Thanks,
Davidlohr