[PATCH tip/locking/core v4 1/6] powerpc: atomic: Make *xchg and *cmpxchg a full barrier

[Paul E. McKenney]

On Tue, Oct 20, 2015 at 11:21:47AM +0200, Peter Zijlstra wrote:
> On Tue, Oct 20, 2015 at 03:15:32PM +0800, Boqun Feng wrote:
> > On Wed, Oct 14, 2015 at 01:19:17PM -0700, Paul E. McKenney wrote:
> > > 
> > > Am I missing something here?  If not, it seems to me that you need
> > > the leading lwsync to instead be a sync.
> > > 
> > > Of course, if I am not missing something, then this applies also to the
> > > value-returning RMW atomic operations that you pulled this pattern from.
> > > If so, it would seem that I didn't think through all the possibilities
> > > back when PPC_ATOMIC_EXIT_BARRIER moved to sync...  In fact, I believe
> > > that I worried about the RMW atomic operation acting as a barrier,
> > > but not as the load/store itself.  :-/
> > > 
> > 
> > Paul, I know this may be difficult, but could you recall why the
> > __futex_atomic_op() and futex_atomic_cmpxchg_inatomic() also got
> > involved into the movement of PPC_ATOMIC_EXIT_BARRIER to "sync"?
> > 
> > I did some search, but couldn't find the discussion of that patch.
> > 
> > I ask this because I recall Peter once bought up a discussion:
> > 
> > https://lkml.org/lkml/2015/8/26/596
> > 
> > Peter's conclusion seems to be that we could(though didn't want to) live
> > with futex atomics not being full barriers.

I have heard of user-level applications relying on unlock-lock being a
full barrier.  So paranoia would argue for the full barrier.

> > Peter, just be clear, I'm not in favor of relaxing futex atomics. But if
> > I make PPC_ATOMIC_ENTRY_BARRIER being "sync", it will also strengthen
> > the futex atomics, just wonder whether such strengthen is a -fix- or
> > not, considering that I want this patch to go to -stable tree.
> 
> So Linus' argued that since we only need to order against user accesses
> (true) and priv changes typically imply strong barriers (open) we might
> want to allow archs to rely on those instead of mandating they have
> explicit barriers in the futex primitives.
> 
> And I indeed forgot to follow up on that discussion.
> 
> So; does PPC imply full barriers on user<->kernel boundaries? If so, its
> not critical to the futex atomic implementations what extra barriers are
> added.
> 
> If not; then strengthening the futex ops is indeed (probably) a good
> thing :-)

I am not seeing a sync there, but I really have to defer to the
maintainers on this one.  I could easily have missed one.

							Thanx, Paul