linux-kernel - Re: [PATCH v2 1/2] spinlock: New spinlock_refcount.h for lockless update of refcount

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Message-ID: <CA+55aFxxUoJYXbN4yqeAbKNKEYpcVMLwopmDuh7sPhT5jnqOEw@mail.gmail.com>
Date:	Sat, 29 Jun 2013 15:47:17 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 1/2] spinlock: New spinlock_refcount.h for lockless
 update of refcount

On Sat, Jun 29, 2013 at 2:58 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> STEP 1:
>
>  - create a new set of config option (say
> "CONFIG_[ARCH_]SPINLOCK_REFCOUNT") that defaults to 'y' and isn't
> actually asked about anywhere.

That "defauls to 'y'" was incomplete/misleading. The
ARCH_SPINLOCK_REFCOUNT config option should default to 'n' (the
example code snippet did that correctly: a bool config option defaults
to 'n' unless stated otherwise).

The SPINLOCK_REFCOUNT in turn _should_ have a "default y" (the sample
snippet I posted didn't have that), but that 'y' is then obviously
normally disabled by the "depends on" logic, so it only takes effect
when ARCH_SPINLOCK_REFCOUNT is set, and none of the spinlock debug
things are enabled.

I hope that was all fairly obvious from the concept (and the real
examples of us already doing things like this with the whole
CONFIG_GENERIC_STRNLEN_USER and CONFIG_DCACHE_WORD_ACCESS examples),
but I thought I'd follow up with an explicit correction, since both
the explanation and sample snippets were somewhat misleading.

                Linus
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