Message-ID: <8cc799ab-ffa1-47f7-6e1d-97488a210f14@huaweicloud.com>
Date:   Tue, 24 Jan 2023 16:11:14 +0100
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Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)
To:     paulmck@kernel.org, Andrea Parri <parri.andrea@gmail.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Jonas Oberhauser <jonas.oberhauser@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>, will <will@kernel.org>,
        "boqun.feng" <boqun.feng@gmail.com>, npiggin <npiggin@gmail.com>,
        dhowells <dhowells@redhat.com>,
        "j.alglave" <j.alglave@ucl.ac.uk>,
        "luc.maranget" <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
        dlustig <dlustig@nvidia.com>, joel <joel@joelfernandes.org>,
        urezki <urezki@gmail.com>,
        quic_neeraju <quic_neeraju@quicinc.com>,
        frederic <frederic@kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
References: <20230120175804.GN2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y8rff3HJ2o9wUyGT@rowland.harvard.edu>
 <20230120192032.GR2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y8r7SBdfuZX/y1cd@rowland.harvard.edu>
 <20230120212037.GW2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y82dWEW4RwclDTGM@rowland.harvard.edu>
 <20230123201659.GA3754540@paulmck-ThinkPad-P17-Gen-1>
 <Y88/5ib7zYl67mcE@rowland.harvard.edu>
 <20230124040611.GD2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y8+8fH52iqQABYs2@andrea>
 <20230124145423.GI2948950@paulmck-ThinkPad-P17-Gen-1>
From:   Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
In-Reply-To: <20230124145423.GI2948950@paulmck-ThinkPad-P17-Gen-1>
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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