Date:   Fri, 24 Feb 2023 10:37:58 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Jonas Oberhauser <jonas.oberhauser@huawei.com>,
        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
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References: <20230224135251.24989-1-jonas.oberhauser@huaweicloud.com>
 <Y/jYm0AZfPHkIalK@rowland.harvard.edu>
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On Fri, Feb 24, 2023 at 10:32:43AM -0500, Alan Stern wrote:
> 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>

Queued for the v6.4 merge window (not the current one), thank you both!

							Thanx, Paul