On Wed, 28 Aug 2013 10:19:37 +0200
Peter Zijlstra <pet...@infradead.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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