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

[Alan Stern]

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++.

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

Alan Stern