Messages in this thread | | | Date | Thu, 30 Nov 2017 08:14:01 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Unlock-lock questions and the Linux Kernel Memory Model |
| |
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
| |