Date:   Sun, 29 Jan 2023 10:44:03 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Andrea Parri <parri.andrea@gmail.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
        will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com,
        npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk,
        luc.maranget@inria.fr, akiyks@gmail.com, dlustig@nvidia.com,
        joel@joelfernandes.org, urezki@gmail.com, quic_neeraju@quicinc.com,
        frederic@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po
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On Sun, Jan 29, 2023 at 06:28:27PM +0100, Andrea Parri wrote:
> > Why can't P3's spin_lock() read from that initial write?
> 
> Mmh, sounds like you want to play with something like below?
> 
>   Andrea
> 
> diff --git a/tools/memory-model/lock.cat b/tools/memory-model/lock.cat
> index 6b52f365d73ac..20c3af4511255 100644
> --- a/tools/memory-model/lock.cat
> +++ b/tools/memory-model/lock.cat
> @@ -74,7 +74,6 @@ flag ~empty UL \ range(critical) as unmatched-unlock
>  
>  (* Allow up to one unmatched LKW per location; more must deadlock *)
>  let UNMATCHED-LKW = LKW \ domain(critical)
> -empty ([UNMATCHED-LKW] ; loc ; [UNMATCHED-LKW]) \ id as unmatched-locks
>  
>  (* rfi for LF events: link each LKW to the LF events in its critical section *)
>  let rfi-lf = ([LKW] ; po-loc ; [LF]) \ ([LKW] ; po-loc ; [UL] ; po-loc)
> @@ -120,8 +119,7 @@ let rf-ru = rfe-ru | rfi-ru
>  let rf = rf | rf-lf | rf-ru
>  
>  (* Generate all co relations, including LKW events but not UL *)
> -let co0 = co0 | ([IW] ; loc ; [LKW]) |
> -	(([LKW] ; loc ; [UNMATCHED-LKW]) \ [UNMATCHED-LKW])
> +let co0 = co0 | ([IW] ; loc ; [LKW])
>  include "cos-opt.cat"
>  let W = W | UL
>  let M = R | W

No idea.  But the following litmus test gets no executions whatsoever,
so point taken about my missing at least one corner case.  ;-)

Adding a spin_unlock() to the end of either process allows both to
run.

One could argue that this is a bug, but one could equally well argue
that if you have a deadlock, you have a deadlock.

Thoughts?

							Thanx, Paul

------------------------------------------------------------------------

C lock

{
}


P0(int *a, int *b, spinlock_t *l)
{
	spin_lock(l);
	WRITE_ONCE(*a, 1);
}

P1(int *a, int *b, spinlock_t *l)
{
	spin_lock(l);
	WRITE_ONCE(*b, 1);
}

exists (a=1 /\ b=1)