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:   Wed, 21 Feb 2018 10:09:00 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:     linux-kernel@...r.kernel.org, <linux-arch@...r.kernel.org>,
        <mingo@...nel.org>, <parri.andrea@...il.com>,
        <will.deacon@....com>, <peterz@...radead.org>,
        <boqun.feng@...il.com>, <npiggin@...il.com>, <dhowells@...hat.com>,
        <j.alglave@....ac.uk>, <luc.maranget@...ia.fr>, <akiyks@...il.com>,
        <nborisov@...e.com>
Subject: Re: [PATCH RFC tools/lkmm 10/12] tools/memory-model: Add a S lock-based
 external-view litmus test

On Tue, 20 Feb 2018, 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

Why do you call this an "S" litmus test?  Isn't ISA2 a better 
description?

> 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
> 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

Aren't these tests supposed to be described in litmus-tests/README?

> 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.

That last line may be premature.  We haven't reached any consensus on 
how RISC-V will handle this.  If RISC-V allows the test then the memory 
model can't forbid it.

Alan

> + *)
> +
> +{}
> +
> +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)
>