linux-kernel - Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

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Date:	Thu, 29 Aug 2013 19:35:41 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Michael Neuling <mikey@...ling.org>,
	Ingo Molnar <mingo@...nel.org>,
	Waiman Long <Waiman.Long@...com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On Thu, Aug 29, 2013 at 7:30 PM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
>
> Or we can keep both completely separate like Linus does on x86.

I did it that way mainly to minimize the patch.

I agree with you that it probably makes sense to layer them the other
way around from what Michael's patch did, iow implement
arch_spin_is_locked() in terms of arch_spin_value_unlocked().

That said, on power, you have that "ACCESS_ONCE()" implicit in the
*type*, not in the code, so an "arch_spinlock_t" is fundamentally
volatile in itself. It's one of the reasons I despise "volatile":
things like volatility are _not_ attributes of a variable or a type,
but of the code in question. Something can be volatile in one context,
but not in another (one context might be locked, for example).

              Linus
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