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Date:   Tue, 24 Jan 2023 17:39:53 +0100
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Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
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To:     paulmck@kernel.org
Cc:     Andrea Parri <parri.andrea@gmail.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Jonas Oberhauser <jonas.oberhauser@huawei.com>,
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        "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: <20230120192032.GR2948950@paulmck-ThinkPad-P17-Gen-1>
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 <Y88/5ib7zYl67mcE@rowland.harvard.edu>
 <20230124040611.GD2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y8+8fH52iqQABYs2@andrea>
 <20230124145423.GI2948950@paulmck-ThinkPad-P17-Gen-1>
 <8cc799ab-ffa1-47f7-6e1d-97488a210f14@huaweicloud.com>
 <20230124162253.GL2948950@paulmck-ThinkPad-P17-Gen-1>
From:   Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
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On 1/24/2023 5:22 PM, Paul E. McKenney wrote:
> I clearly recall some
> store-based lack of ordering after a grace period from some years back,
> and am thus far failing to reproduce it.
>
> And here is another attempt that herd7 actually does allow.
>
> So what did I mess up this time?  ;-)
>
> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> C C-srcu-observed-4
>
> (*
>   * Result: Sometimes
>   *
>   * The Linux-kernel implementation is suspected to forbid this.
>   *)
>
> {}
>
> P0(int *x, int *y, int *z, struct srcu_struct *s)
> {
> 	int r1;
>
> 	r1 = srcu_read_lock(s);
> 	WRITE_ONCE(*y, 2);
> 	WRITE_ONCE(*x, 1);
> 	srcu_read_unlock(s, r1);
> }
>
> P1(int *x, int *y, int *z, struct srcu_struct *s)
> {
> 	int r1;
>
> 	WRITE_ONCE(*y, 1);
> 	synchronize_srcu(s);
> 	WRITE_ONCE(*z, 2);
> }
>
> P2(int *x, int *y, int *z, struct srcu_struct *s)
> {
> 	WRITE_ONCE(*z, 1);
> 	smp_store_release(x, 2);
> }
>
> exists (x=1 /\ y=1 /\ z=1)

I think even if you implement the unlock as mb() followed by some store 
that is read by the gp between mb()s, this would still be allowed.

I have already forgotten the specifics, but I think the power model 
allows certain stores never propagating somewhere?
If z=2,z=1,x=2 never propagate to P0, you might start by executing P0, 
then P1, and then P2 at which point the memory system decides that x=1 
overwrites x=2, and the latter simply doesn't propagate anywhere.

(I'll let anyone who has the model at hand correct me on this, because I 
have to take a walk now).

Have fun, jonas