Date:   Thu, 26 Jan 2023 11:36:08 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc:     paulmck@kernel.org, parri.andrea@gmail.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
Message-ID: <Y9Kr+GntQyGKPH3K@rowland.harvard.edu>
References: <20230126134604.2160-1-jonas.oberhauser@huaweicloud.com>
 <20230126134604.2160-3-jonas.oberhauser@huaweicloud.com>
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On Thu, Jan 26, 2023 at 02:46:04PM +0100, Jonas Oberhauser wrote:
> As stated in the documentation and implied by its name, the ppo
> (preserved program order) relation is intended to link po-earlier
> to po-later instructions under certain conditions.  However, a
> corner case currently allows instructions to be linked by ppo that
> are not executed by the same thread, i.e., instructions are being
> linked that have no po relation.
> 
> This happens due to the mb/strong-fence relations, which (as one
> case) provide order when locks are passed between threads followed
> by an smp_mb__after_unlock_lock() fence.  This is illustrated in
> the following litmus test (as can be seen when using herd7 with
> `doshow ppo`):
> 
> P0(int *x, int *y)
> {
>     spin_lock(x);
>     spin_unlock(x);
> }
> 
> P1(int *x, int *y)
> {
>     spin_lock(x);
>     smp_mb__after_unlock_lock();
>     *y = 1;
> }
> 
> The ppo relation will link P0's spin_lock(x) and P1's *y=1, because
> P0 passes a lock to P1 which then uses this fence.
> 
> The patch makes ppo a subrelation of po by eliminating this possibility
> from mb (but not strong-fence) and relying explicitly on mb|gp instead
> of strong-fence when defining ppo.
> 
> Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
> ---

This changes the meaning of the fence relation, which is used in 
w-pre-bounded, w-post-bounded, ww-vis, wr-vis, and rw-xbstar.  Have you 
checked that they won't be affected by the change?

>  tools/memory-model/linux-kernel.cat | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> index 6e531457bb73..815fdafacaef 100644
> --- a/tools/memory-model/linux-kernel.cat
> +++ b/tools/memory-model/linux-kernel.cat
> @@ -36,7 +36,9 @@ let wmb = [W] ; fencerel(Wmb) ; [W]
>  let mb = ([M] ; fencerel(Mb) ; [M]) |
>  	([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) |
>  	([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) |
> -	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
> +	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M])
> +let gp = po ; [Sync-rcu | Sync-srcu] ; po?
> +let strong-fence = mb | gp |
>  (*
>   * Note: The po-unlock-lock-po relation only passes the lock to the direct
>   * successor, perhaps giving the impression that the ordering of the
> @@ -50,10 +52,9 @@ let mb = ([M] ; fencerel(Mb) ; [M]) |
>   *)
>  	([M] ; po-unlock-lock-po ;
>  		[After-unlock-lock] ; po ; [M])
> -let gp = po ; [Sync-rcu | Sync-srcu] ; po?
> -let strong-fence = mb | gp
>  
> -let nonrw-fence = strong-fence | po-rel | acq-po
> +

Extra blank line.

> +let nonrw-fence = mb | gp | po-rel | acq-po
>  let fence = nonrw-fence | wmb | rmb
>  let barrier = fencerel(Barrier | Rmb | Wmb | Mb | Sync-rcu | Sync-srcu |
>  		Before-atomic | After-atomic | Acquire | Release |
> -- 
> 2.17.1

Alan