Messages in this thread | | | Date | Thu, 30 Nov 2017 16:41:05 +0000 | From | Will Deacon <> | Subject | Re: Unlock-lock questions and the Linux Kernel Memory Model |
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On Thu, Nov 30, 2017 at 08:14:01AM -0800, Paul E. McKenney wrote: > 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?
We discussed it, but rejected it mainly because of concerns that there could be RmW operations that don't necessarily have an order-inducing dependency in all scenarios. I think the case that was batted around was a saturating add implemented using cmpxchg.
Will
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