Re: [PATCH v5 tip/core/locking 5/7]Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK

[Josh Triplett]

On Tue, Dec 10, 2013 at 06:25:28PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 10, 2013 at 09:12:47AM -0800, Paul E. McKenney wrote:
> > > The way I read the above it says that you need
> > > smp_mb__after_unlock_lock() when the UNLOCK and LOCK are on the same
> > > variable. That doesn't make sense, I thought that was the one case we
> > > all agreed on it would indeed be a full barrier without extra trickery.
> > 
> > On x86, sure, but smp_mb__after_unlock_lock() is nothingness on x86
> > anyway.  Other architectures might benefit from requiring that the
> > smp_mb__after_unlock_lock() be used in this case.
> 
> Confused, UNLOCK X, LOCK X, must always be fully serializing. That's the
> entire purpose of the thing.
> 
> The only place you can go play games (and clearly we are going there) is
> when the UNLOCK and LOCK are on different variables.

That would certainly be a good assumption to preserve, and it would
eliminate most of the need for smp_mb__after_unlock_lock().

- Josh Triplett
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