linux-kernel - Re: [RFC][PATCH 2/3] locking: Annotate spin_unlock

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Date:	Tue, 24 May 2016 09:17:13 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Will Deacon <will.deacon@....com>,
	Boqun Feng <boqun.feng@...il.com>,
	Waiman Long <Waiman.Long@....com>, Tejun Heo <tj@...nel.org>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Patrick McHardy <kaber@...sh.net>,
	David Miller <davem@...emloft.net>,
	Oleg Nesterov <oleg@...hat.com>,
	netfilter-devel@...r.kernel.org,
	Sasha Levin <sasha.levin@...cle.com>, hofrat@...dl.org
Subject: Re: [RFC][PATCH 2/3] locking: Annotate spin_unlock_wait() users

On Tue, May 24, 2016 at 7:27 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> spin_unlock_wait() has an unintuitive 'feature' in that it doesn't
> fully serialize against the spin_unlock() we've waited on.

NAK.

We don't start adding more of this "after_ctrl_dep" crap.

It's completely impossible to understand, and even people who have
been locking experts have gotten it wrong.

So it is *completely* unacceptable to have it in drivers.

This needs to be either hidden inside the basic spinlock functions,
_or_ it needs to be a clear and unambiguous interface. Anything that
starts talking about control dependencies is not it.

Note that this really is about naming and use, not about
implementation. So something like "spin_sync_after_unlock_wait()" is
acceptable, even if the actual _implementation_ were to be exactly the
same as the "after_ctrl_dep()" crap.

The difference is that one talks about incomprehensible implementation
details that nobody outside of the person who *implemented* the
spinlock code is supposed to understand (and seriously, I have my
doubts even the spinlock implementer understands it, judging by the
last time this happened), and the other is a much simpler semantic
guarantee.

So don't talk about "acquire". And most certainly don't talk about
"control dependencies". Not if we end up having things like *drivers*
using this like in this example libata.

                Linus