Messages in this thread | | | Date | Wed, 25 Jan 2023 21:36:11 +0100 | From | Andrea Parri <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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> > 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
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