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 08:37:30 +1000 |
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On Mon, 2015-07-13 at 17:54 +0200, Peter Zijlstra wrote:
> That said, I don't think this could even happen on PPC because we have > load_acquire and store_release, this means that: > > *A = a > lwsync > store_release M > load_acquire N > lwsync > *B = b > > And since the store to M is wrapped inside two lwsync there must be > strong store order, and because the load from N is equally wrapped in > two lwsyncs there must also be strong load order. > > In fact, no store/load can cross from before the first lwsync to after > the latter and the other way around. > > So in that respect it does provide full load-store ordering. What it > does not provide is order for M and N, nor does it provide transitivity, > but looking at our documentation I'm not at all sure we guarantee that > in any case.
The problem is if you have a load after the second lwsync, that load can go up pass the store release. This has caused issues when N or M is what you are trying to order against. That's why we had to add a sync to spin_is_locked or similar.
Ben.
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