From mboxrd@z Thu Jan  1 00:00:00 1970
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount
Date: Fri, 30 Aug 2013 12:30:57 +1000
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	Waiman Long <Waiman.Long@hp.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jeff Layton <jlayton@redhat.com>,
	Miklos Szeredi <mszeredi@suse.cz>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andi Kleen <andi@firstfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@hp.com>,
	"Norton, Scott J" <scott.norton@hp.com>
To: Michael Neuling <mikey@neuling.org>
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On Fri, 2013-08-30 at 12:06 +1000, Michael Neuling wrote:

> powerpc patch below. I'm using arch_spin_is_locked() to implement
> arch_spin_value_unlocked().

>  
> +static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> +{
> +	return !arch_spin_is_locked(&lock);
> +}
> +

Arguably, it should be done the other way around :-) 

arch_spin_value_unlocked semantics is to basically operate on an already
read copy of the value, while arch_spin_is_locked() has ACCESS_ONE
semantics on *top* of that.

Or we can keep both completely separate like Linus does on x86.

Cheers,
Ben.