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
>
From 1585434595230387843@xxx Wed Nov 29 20:44:13 +0000 2017
X-GM-THRID: 1585255508732072548
X-Gmail-Labels: Inbox,Category Forums,HistoricalUnread