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