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

[Boqun Feng]

On Tue, Oct 20, 2015 at 02:28:35PM -0700, Paul E. McKenney wrote:
> 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.
> 

Understood.

So a full barrier on one side of these operations is enough, I think.
IOW, there is no need to strengthen these operations.

> > > 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 :-)

Peter, that's probably a good thing, but I'm not that familiar with
futex right now, so I won't touch that part if unnecessary in this
series.

Regards,
Boqun

> 
> 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
>

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