linux-kernel - Re: [PATCH RFC v2 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

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Date:	Wed, 28 Aug 2013 08:59:57 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Alexander Fyodorov <halcy@...dex.ru>,
	Waiman Long <waiman.long@...com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH RFC v2 1/2] qspinlock: Introducing a 4-byte queue
 spinlock implementation

On Wed, 28 Aug 2013 10:19:37 +0200
Peter Zijlstra <peterz@...radead.org> wrote:


> > An unlock followed by a lock needs to act like a full barrier, but there
> > is no requirement that a lock or unlock taken separately act like a
> > full barrier.
> 
> But that is already a property of the acquisition/release barrier.

As I mentioned in my fixes for the -rt swait barrier patches I sent.
Spin locks only prevent leaks out of the critical section. It does not
guarantee leaks into the critical section, thus:


	A = 1

	spin_lock()



	spin_unlock()

	B = C

Can turn into:


	(A = 1)

	spin_lock()

	load C

	store 1 into A

	spin_unlock()

	B = C

This shows that a spin_lock()/unlock() combo is not equivalent to a
mb(). But as Paul has mentioned, if we had:

	A = 1

	spin_unlock()

	spin_lock()

	B = C

That would be equivalent to

	A = 1

	mb()

	B = C

as the unlock prevents leaks going past it, and lock prevents leaks
going before it.

-- Steve
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