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

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Date:	Sat, 29 Jun 2013 16:23:10 -0400
From:	Waiman Long <waiman.long@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 06/29/2013 01:45 PM, Linus Torvalds wrote:
> 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.

I was concern that people might want to have the same behavior even when 
spinlock debugging was  on. Apparently, this is not really needed. Now I 
can just disable the optimization and fall back to the old path when 
spinlock debugging is on.

> 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.

Yes, I can put the current implementation into 
asm-generic/spinlock_refcount.h. Now I need to put an 
asm/spinlock_refcount.h into every arch's include/asm directory. Right? 
I don't think there is a mechanism in the build script to create a 
symlink from asm to generic-asm when a header file is missing. Is it the 
general rule that we should have a linux/spinlock_refcount.h that 
include asm/spinlock_refcount.h instead of including 
asm/spinlock_refcount.h directly?

> 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

Thank for the comment. I will try to come up with a version that is 
acceptable to all stakeholders.

Regards,
Longman

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