Date:   Fri, 24 Feb 2023 10:32:43 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@huawei.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 v3] tools/memory-model: Make ppo a subrelation of po
Message-ID: <Y/jYm0AZfPHkIalK@rowland.harvard.edu>
References: <20230224135251.24989-1-jonas.oberhauser@huaweicloud.com>
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On Fri, Feb 24, 2023 at 02:52:51PM +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/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 letting fence contribute
> to ppo only in case the fence links events of the same thread.
> 
> Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
> ---
>  tools/memory-model/linux-kernel.cat | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> index cfc1b8fd46da..adf3c4f41229 100644
> --- a/tools/memory-model/linux-kernel.cat
> +++ b/tools/memory-model/linux-kernel.cat
> @@ -82,7 +82,7 @@ let rwdep = (dep | ctrl) ; [W]
>  let overwrite = co | fr
>  let to-w = rwdep | (overwrite & int) | (addr ; [Plain] ; wmb)
>  let to-r = (addr ; [R]) | (dep ; [Marked] ; rfi)
> -let ppo = to-r | to-w | fence | (po-unlock-lock-po & int)
> +let ppo = to-r | to-w | (fence & int) | (po-unlock-lock-po & int)
>  
>  (* Propagation: Ordering from release operations and strong fences. *)
>  let A-cumul(r) = (rfe ; [Marked])? ; r

Acked-by: Alan Stern <stern@rowland.harvard.edu>