Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Jonas Oberhauser
2023-01-25T21:12:22Z

On 1/25/2023 9:36 PM, Andrea Parri wrote:
>>> Why do you want the implementation to forbid it? The pattern of the
>>> litmus test resembles 3+3W, and you don't care whether the kernel allows
>>> that pattern. Do you?
>> Jonas asked a similar question, so I am answering you both here.
>>
>> With (say) a release-WRITE_ONCE() chain implementing N+2W for some
>> N, it is reasonably well known that you don't get ordering, hardware
>> support otwithstanding. After all, none of the Linux kernel, C, and C++
>> memory models make that guarantee. In addition, the non-RCU barriers
>> and accesses that you can use to create N+2W have been in very wide use
>> for a very long time.
>>
>> Although RCU has been in use for almost as long as those non-RCU barriers,
>> it has not been in wide use for anywhere near that long. So I cannot
>> be so confident in ruling out some N+2W use case for RCU.
> Did some archeology... the pattern, with either RCU sync plus a release
> or with two full fences plus a release, was forbidden by "ancient LKMM":
> the relevant changes were described in
>
> https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/LWNLinuxMM/WeakModel.html#Coherence%20Point%20and%20RCU
>
> Andrea

Fascinating! It says there "But the weak model allows it, as required"
-- what does "as required" mean? Just "as required by dropping the
constraint"?

Is there still a notion of "strong model" and "weak model", or was the
strong model dropped?

jonas