On Thu, Aug 10, 2017 at 04:12:13PM +0800, Boqun Feng wrote: > > Or is the reason this doesn't work on PPC that its RCpc?
So that :-) > Here is an example why PPC needs a sync() before the cmpxchg(): > > https://marc.info/?l=linux-kernel&m=144485396224519&w=2 > > and Paul Mckenney's detailed explanation about why this could happen: > > https://marc.info/?l=linux-kernel&m=144485909826241&w=2 > > (Somehow, I feel like he was answering to a similar question question as > you ask here ;-)) Yes, and I had vague memories of having gone over this before, but couldn't quickly find things. Thanks! > And I think aarch64 doesn't have a problem here because it is "(other) > multi-copy atomic". Will? Right, its the RCpc vs RCsc thing. The ARM64 release is as you say multi-copy atomic, whereas the PPC lwsync is not. This still leaves us with the situation that we need an smp_mb() between smp_store_release() and a possibly failing cmpxchg() if we want to guarantee the cmpxchg()'s load comes after the store-release.
