Re: [PATCH RFC v2 1/2] qspinlock: Introducing a 4-byte queuespinlock implementation

[Steven Rostedt]

On Wed, 28 Aug 2013 10:19:37 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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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