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Date: Mon, 2 Nov 2015 23:12:22 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Paul Turner <pjt@google.com>
Cc: Ingo Molnar <mingo@kernel.org>, Oleg Nesterov <oleg@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>, boqun.feng@gmail.com,
        Jonathan Corbet <corbet@lwn.net>, mhocko@kernel.org,
        dhowells@redhat.com, Linus Torvalds <torvalds@linux-foundation.org>,
        will.deacon@arm.com
Subject: Re: [PATCH 2/4] sched: Document Program-Order guarantees
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On Mon, Nov 02, 2015 at 02:09:20PM -0800, Paul Turner wrote:
> If we went this route, we could do something like:
> 
> + * So in this case the scheduler does not provide an obvious full barrier; but
> + * the smp_store_release() in finish_lock_switch(), paired with the control-dep
> + * and smp_rmb() in try_to_wake_up() form a release-acquire pair and fully
> + * order things between CPU0 and CPU1.
> 
> Instead of having this, which is complete, but hard to synchronize
> with the points at which it actually matters.  Just use acquire and
> release above, then at the actual site, e.g. in try_to_wake_up()
> document how we deliver the acquire required by the higher level
> documentation/requirements.

Right, which was most of the point of trying to introduce
smp_cond_acquire(), abstract out the tricky cond-dep and rmb trickery
so we can indeed talk about release+acquire like normal people ;-)