On Mon, Jul 13, 2015 at 01:16:42PM -0700, Paul E. McKenney wrote:
> On Mon, Jul 13, 2015 at 03:41:53PM -0400, Peter Hurley wrote:
> > > Does that answer the question, or am I missing the point?
> >
> > Yes, it shows that smp_mb__after_unlock_lock() has no purpose, since it
> > is defined only for PowerPC and your test above just showed that for
> > the sequence
The only purpose is to provide transitivity, but the documentation fails
to explicitly call that out.
> >
> > store a
> > UNLOCK M
> > LOCK N
> > store b
> >
> > a and b is always observed as an ordered pair {a,b}.
>
> Not quite.
>
> This is instead the sequence that is of concern:
>
> store a
> unlock M
> lock N
> load b
So its late and that table didn't parse, but that should be ordered too.
The load of b should not be able to escape the lock N.
If only because LWSYNC is a valid RMB and any LOCK implementation must
load the lock state to observe it unlocked.
> > Additionally, the assertion in Documentation/memory_barriers.txt that
> > the sequence above can be reordered as
> >
> > LOCK N
> > store b
> > store a
> > UNLOCK M
> >
> > is not true on any existing arch in Linux.
>
> It was at one time and might be again.
What would be required to make this true? I'm having a hard time seeing
how things can get reordered like that.
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