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

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Date:   Wed, 25 Jan 2023 21:36:11 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, will <will@...nel.org>,
        "boqun.feng" <boqun.feng@...il.com>, npiggin <npiggin@...il.com>,
        dhowells <dhowells@...hat.com>,
        "j.alglave" <j.alglave@....ac.uk>,
        "luc.maranget" <luc.maranget@...ia.fr>, akiyks <akiyks@...il.com>,
        dlustig <dlustig@...dia.com>, joel <joel@...lfernandes.org>,
        urezki <urezki@...il.com>,
        quic_neeraju <quic_neeraju@...cinc.com>,
        frederic <frederic@...nel.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)

> > 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