Messages in this thread | | | Date | Tue, 14 Jun 2016 13:52:53 +0800 | From | Boqun Feng <> | Subject | Re: [PATCH] locking/qspinlock: Use atomic_sub_return_release in queued_spin_unlock |
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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) <peterz@infradead.org> > > Signed-off-by: Pan Xinhui <xinhui.pan@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 [unhandled content-type:application/pgp-signature] | |