linux-kernel - Re: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S lock-based external-view litmus test

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Date:   Thu, 22 Feb 2018 11:23:49 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, stern@...land.harvard.edu,
        parri.andrea@...il.com, will.deacon@....com, peterz@...radead.org,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com, nborisov@...e.com,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S
 lock-based external-view litmus test

On Tue, Feb 20, 2018 at 03:25:10PM -0800, Paul E. McKenney wrote:
> From: Alan Stern <stern@...land.harvard.edu>
> 
> This commit adds a litmus test in which P0() and P1() form a lock-based S
> litmus test, with the addition of P2(), which observes P0()'s and P1()'s
> accesses with a full memory barrier but without the lock.  This litmus
> test asks whether writes carried out by two different processes under the
> same lock will be seen in order by a third process not holding that lock.
> The answer to this question is "yes" for all architectures supporting

Hmm.. it this true? Our spin_lock() is RCpc because of PowerPC, so
spin_lock()+spin_unlock() pairs don't provide transitivity, and that's
why we have smp_mb__after_unlock_lock(). Is there something I'm missing?
Or there is an upcomming commit to switch PowerPC's lock implementation?

[Cc ppc maintainers]

Regards,
Boqun

> the Linux kernel, but is "no" according to the current version of LKMM.
> 
> A patch to LKMM is under development.
> 
> Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  .../ISA2+pooncelock+pooncelock+pombonce.litmus     | 41 ++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
>  create mode 100644 tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
> 
> diff --git a/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus b/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
> new file mode 100644
> index 000000000000..7a39a0aaa976
> --- /dev/null
> +++ b/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
> @@ -0,0 +1,41 @@
> +C ISA2+pooncelock+pooncelock+pombonce.litmus
> +
> +(*
> + * Result: Sometimes
> + *
> + * This test shows that the ordering provided by a lock-protected S
> + * litmus test (P0() and P1()) are not visible to external process P2().
> + * This is likely to change soon.
> + *)
> +
> +{}
> +
> +P0(int *x, int *y, spinlock_t *mylock)
> +{
> +	spin_lock(mylock);
> +	WRITE_ONCE(*x, 1);
> +	WRITE_ONCE(*y, 1);
> +	spin_unlock(mylock);
> +}
> +
> +P1(int *y, int *z, spinlock_t *mylock)
> +{
> +	int r0;
> +
> +	spin_lock(mylock);
> +	r0 = READ_ONCE(*y);
> +	WRITE_ONCE(*z, 1);
> +	spin_unlock(mylock);
> +}
> +
> +P2(int *x, int *z)
> +{
> +	int r1;
> +	int r2;
> +
> +	r2 = READ_ONCE(*z);
> +	smp_mb();
> +	r1 = READ_ONCE(*x);
> +}
> +
> +exists (1:r0=1 /\ 2:r2=1 /\ 2:r1=0)
> -- 
> 2.5.2
> 

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