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 :) Dan > Thanx, Paul >