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

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Message-ID: <51CB9233.7020508@hp.com>
Date:	Wed, 26 Jun 2013 21:15:31 -0400
From:	Waiman Long <waiman.long@...com>
To:	Andi Kleen <andi@...stfloor.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@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.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 06/26/2013 09:06 PM, Andi Kleen wrote:
>> In the uncontended case, doing spin_unlock_wait will be similar to
>> spin_can_lock. This, when combined with a cmpxchg, is still faster
>> than doing 2 atomic operations in spin_lock/spin_unlock.
> I'm totally against any new users of spin_unlock_wait()
>
> It has bizarre semantics, most likely will make various
> lock optimizations impossible, it's race condition hell
> for most users etc.
>
> spin_can_lock() is not quite as bad has a lot of the similar problems.
>
>> BTW, spin_can_lock is just the negation of spin_is_locked.
> e.g. with elision it's not.
>
> -Andi

OK, it is about Haswell's lock elision feature. I will see what I can do 
to remove those problematic function calls.

Regards,
Longman
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