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

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Date:   Tue, 24 Jan 2023 16:11:14 +0100
From:   Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To:     paulmck@...nel.org, Andrea Parri <parri.andrea@...il.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        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)



On 1/24/2023 3:54 PM, Paul E. McKenney wrote:
> On Tue, Jan 24, 2023 at 12:09:48PM +0100, Andrea Parri wrote:
>>> There is the one below, but I am (1) not sure that I have it right,
>>> (2) not immediately certain that the Linux-kernel implementation would
>>> forbid it, (3) not immediately sure that it should be forbidden.
>>>
>>> In the meantime, thoughts?
>> As it stands, P0 to completion, then P1 to completion, then P2 to
>> completion should meet the "exists" clause; I guess we want "x=1"
>> in the clause (or the values of the stores to "x" exchanged).
> OK, so I still don't have it right.  ;-)
>
> Make that x=1.  I think.
>

If it is x=1, why doesn't LKMM forbid it?
Because T1:y=1 is read by T1 before the GP, the whole CS is before the 
GP, i.e.,

srcu_read_unlock(s, r1);  ->rcu-order synchronize_srcu(s);

The GP is furthermore po;prop;strong-fence;prop;po ordered before the 
unlock, which you can shuffle around to get
    Wx=2  ->prop;po;rcu-order;po ;  prop;strong-fence  Wx=2
or
    Wx=2  ->rb  Wx=2
which is forbidden because rb is irreflexive.

Right?

jonas