On Mon, Jun 13, 2016 at 12:45:23PM -0700, Davidlohr Bueso wrote: > On Fri, 03 Jun 2016, Pan Xinhui wrote: > > > The existing version uses a heavy barrier while only release semantics > > is required. So use atomic_sub_return_release instead. > > > > Suggested-by: Peter Zijlstra (Intel) <pet...@infradead.org> > > Signed-off-by: Pan Xinhui <xinhui....@linux.vnet.ibm.com> > > I just noticed this change in -tip and, while I know that saving a barrier > in core spinlock paths is perhaps a worthy exception, I cannot help but > wonder if this is the begging of the end for smp__{before,after}_atomic().
This is surely a good direction I think, that is using _acquire and _release primitives to replace those barriers. However, I think we should do this carefully, because the _acquire and _release primitives are RCpc because they are on PPC, IOW, a ACQUIRE and RELEASE pair is not a full barrier nor provides global transivity. I'm worried about there are some users depending on the full-barrier semantics, which means we must audit each use carefully before we make the change. Besides, if we want to do the conversion, we'd better have _acquire and _release variants for non-value-returning atomic operations. I remember you were working on those variants. How is that going? Regards, Boqun
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