Messages in this thread | | | Date | Tue, 14 Jul 2015 00:15:03 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() |
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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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