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

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Message-ID: <20130627010658.GL6123@two.firstfloor.org>
Date:	Thu, 27 Jun 2013 03:06:58 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	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>
Subject: Re: [PATCH v2 1/2] spinlock: New spinlock_refcount.h for lockless
 update of refcount

> 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

-- 
ak@...ux.intel.com -- Speaking for myself only.
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