linux-kernel - Re: [PATCH 2/4] sched: Document Program-Order guarantees

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Message-ID: <20151102221222.GB17308@twins.programming.kicks-ass.net>
Date:	Mon, 2 Nov 2015 23:12:22 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Turner <pjt@...gle.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	boqun.feng@...il.com, Jonathan Corbet <corbet@....net>,
	mhocko@...nel.org, dhowells@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	will.deacon@....com
Subject: Re: [PATCH 2/4] sched: Document Program-Order guarantees

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 ;-)
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