Date:   Tue, 24 Jan 2023 08:22:53 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc:     Andrea Parri <parri.andrea@gmail.com>,
        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>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)
Message-ID: <20230124162253.GL2948950@paulmck-ThinkPad-P17-Gen-1>
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On Tue, Jan 24, 2023 at 04:11:14PM +0100, Jonas Oberhauser wrote:
> 
> 
> 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?

Yes according to herd7, hence the "I think".  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)