Re: Unlock-lock questions and the Linux Kernel Memory Model

[Will Deacon <will.deacon@arm.com>]

On Thu, Nov 30, 2017 at 08:14:01AM -0800, Paul E. McKenney wrote:
> On Thu, Nov 30, 2017 at 10:20:02AM -0500, Alan Stern wrote:
> > On Wed, 29 Nov 2017, Daniel Lustig wrote:
> > 
> > > On 11/29/2017 12:42 PM, Paul E. McKenney wrote:
> > > > On Wed, Nov 29, 2017 at 02:53:06PM -0500, Alan Stern wrote:
> > > >> On Wed, 29 Nov 2017, Peter Zijlstra wrote:
> > > >>
> > > >>> On Wed, Nov 29, 2017 at 11:04:53AM -0800, Daniel Lustig wrote:
> > > >>>
> > > >>>> While we're here, let me ask about another test which isn't directly
> > > >>>> about unlock/lock but which is still somewhat related to this
> > > >>>> discussion:
> > > >>>>
> > > >>>> "MP+wmb+xchg-acq" (or some such)
> > > >>>>
> > > >>>> {}
> > > >>>>
> > > >>>> P0(int *x, int *y)
> > > >>>> {
> > > >>>>         WRITE_ONCE(*x, 1);
> > > >>>>         smp_wmb();
> > > >>>>         WRITE_ONCE(*y, 1);
> > > >>>> }
> > > >>>>
> > > >>>> P1(int *x, int *y)
> > > >>>> {
> > > >>>>         r1 = atomic_xchg_relaxed(y, 2);
> > > >>>>         r2 = smp_load_acquire(y);
> > > >>>>         r3 = READ_ONCE(*x);
> > > >>>> }
> > > >>>>
> > > >>>> exists (1:r1=1 /\ 1:r2=2 /\ 1:r3=0)
> > > >>>>
> > > >>>> C/C++ would call the atomic_xchg_relaxed part of a release sequence
> > > >>>> and hence would forbid this outcome.
> > > >>>
> > > >>> That's just weird. Either its _relaxed, or its _release. Making _relaxed
> > > >>> mean _release is just daft.
> > > >>
> > > >> The C11 memory model specifically allows atomic operations to be 
> > > >> interspersed within a release sequence.  But it doesn't say why.
> > > > 
> > > > The use case put forward within the committee is for atomic quantities
> > > > with mode bits.  The most frequent has the atomic quantity having
> > > > lock-like properties, in which case you don't want to lose the ordering
> > > > effects of the lock handoff just because a mode bit got set or cleared.
> > > > Some claim to actually use something like this, but details have not
> > > > been forthcoming.
> > > > 
> > > > I confess to being a bit skeptical.  If the mode changes are infrequent,
> > > > the update could just as well be ordered.
> > > 
> > > Aren't reference counting implementations which use memory_order_relaxed
> > > for incrementing the count another important use case?  Specifically,
> > > the synchronization between a memory_order_release decrement and the
> > > eventual memory_order_acquire/consume free shouldn't be interrupted by
> > > other (relaxed) increments and (release-only) decrements that happen in
> > > between.  At least that's my understanding of this use case.  I wasn't
> > > there when the C/C++ committee decided this.
> > > 
> > > > That said, Daniel, the C++ memory model really does require that the
> > > > above litmus test be forbidden, my denigration of it notwithstanding.
> > > 
> > > Yes I agree, that's why I'm curious what the Linux memory model has
> > > in mind here :)
> > 
> > Bear in mind that the litmus test above uses xchg, not increment or 
> > decrement.  This makes a difference as far as the LKMM is concerned, 
> > even if not for C/C++.
> 
> Finally remembering this discussion...  Yes, xchg is special.  ;-)
> 
> Will, are there plans to bring this sort of thing before the standards
> committee?

We discussed it, but rejected it mainly because of concerns that there could
be RmW operations that don't necessarily have an order-inducing dependency
in all scenarios. I think the case that was batted around was a saturating
add implemented using cmpxchg.

Will