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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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?

> (Also, technically speaking, the litmus test doesn't have any release 
> operations, so no release sequence...)

True!  But if you translated it into C11, you would probably turn the
smp_wmb() followed by write into a store release, which would get you
a release sequence.

							Thanx, Paul