linux-kernel - Re: XDP socket rings, and LKMM litmus tests

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Date:   Tue, 2 Mar 2021 12:41:57 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        stern@...land.harvard.edu, parri.andrea@...il.com,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com, dlustig@...dia.com,
        joel@...lfernandes.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: XDP socket rings, and LKMM litmus tests

On Tue, Mar 02, 2021 at 09:24:04PM +0100, Björn Töpel wrote:
> On Tue, 2 Mar 2021 at 20:57, Paul E. McKenney <paulmck@...nel.org> wrote:
> >
> > On Tue, Mar 02, 2021 at 07:46:27PM +0100, Björn Töpel wrote:
> 
> [...]
> 
> >
> > Before digging in too deeply, does the following simplification
> > still capture your intent?
> >
> 
> Thanks for having a look, Paul!
> 
> > P0(int *prod, int *cons, int *data)
> > {
> >     int p;
> >     int cond = 0;
> >
> >     p = READ_ONCE(*prod);
> >     if (p == READ_ONCE(*cons))
> >             cond = 1;
> 
> With this, yes!
> 
> >     if (cond) {
> >         smp_mb();
> >         WRITE_ONCE(*data, 1);
> >         smp_wmb();
> >         WRITE_ONCE(*prod, p ^ 1);
> >     }
> > }
> >
> > P1(int *prod, int *cons, int *data)
> > {
> >     int c;
> >     int d = -1;
> >     int cond = 0;
> >
> >     c = READ_ONCE(*cons);
> >     if (READ_ONCE(*prod) == c)
> >             cond = 1;
> 
> Hmm, this would not be the correct state transition.
> 
> c==1 && p==1 would set cond to 1, right?
> 
> I would agree with:
>   c = READ_ONCE(*cons);
>   if (READ_ONCE(*prod) != c)

Right you are!

With that, it looks to me like LKMM is OK with removing the smp_mb().
My guess is that the issue is that LKMM confines the effect of control
dependencies to a single "if" statement, hence my reworking of your
original.

							Thanx, Paul

> >
> >     if (cond == 1) {
> >         smp_rmb();
> >         d = READ_ONCE(*data);
> >         smp_mb();
> >         WRITE_ONCE(*cons, c ^ 1);
> >     }
> > }
> >
> >                                                         Thanx, Paul
> >
> 
> [...]
> 
> Björn