Messages in this thread | | | Subject | Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() | From | Benjamin Herrenschmidt <> | Date | Tue, 14 Jul 2015 08:43:44 +1000 |
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On Tue, 2015-07-14 at 00:15 +0200, Peter Zijlstra wrote:
> > 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.
What happens is that the load passes the store conditional, though it doesn't pass the load with reserve. However, both store A and unlock M being just stores with an lwsync, can pass a load, so they can pass the load with reserve. And thus inside the LL/SC loop, our store A has passed our load B.
> > > 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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