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

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Date:	Wed, 26 Jun 2013 21:24:32 -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 09:15 PM, Waiman Long wrote:
> 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.

It will be hard to know what changes will be needed without knowing the 
exact semantics of the spinlock functions with lock elision. Can you 
explain a little more what bizarre semantics you are referring to?

regards,
Longman
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