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

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Message-ID: <CA+55aFzo5Jy064pbvEZh5cg_RJiSxe1R+tSgByQTs4rc=4SN3Q@mail.gmail.com>
Date:	Sat, 29 Jun 2013 10:45:16 -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>
Subject: Re: [PATCH v2 1/2] spinlock: New spinlock_refcount.h for lockless
 update of refcount

Sorry for not commenting earlier, I was traveling and keeping email to
a minimum..

On Wed, Jun 26, 2013 at 10:43 AM, Waiman Long <Waiman.Long@...com> wrote:
> This patch introduces a new spinlock_refcount.h header file to be
> included by kernel code that want to do a lockless update of reference
> count protected by a spinlock.

So I really like the concept, but the implementation is a mess, and
tries to do too much, while actually achieving too little.

I do not believe you should care about debug spinlocks at all, and
just leave them be. Have a simple fallback code that defaults to
regular counts and spinlocks, and have any debug cases just use that.

But more importantly, I think this needs to be architecture-specific,
and using <linux/spinlock_refcount.h> to try to do some generic 64-bit
cmpxchg() version is a bad bad idea.

We have several architectures coming up that have memory transaction
support, and the "spinlock with refcount" is a perfect candidate for a
transactional memory implementation. So when introducing a new atomic
like this that is very performance-critical and used for some very
core code, I really think architectures would want to make their own
optimized versions.

These things should also not be inlined, I think.

So I think the concept is good, but I think the implementation needs
more thought.

                       Linus
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