From: "Paul E. McKenney" <paulmck@kernel.org>
To: "Björn Töpel" <bjorn.topel@gmail.com>
Cc: bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
stern@rowland.harvard.edu, parri.andrea@gmail.com,
"Will Deacon" <will@kernel.org>,
"Peter Zijlstra" <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,
"Toke Høiland-Jørgensen" <toke@redhat.com>,
"Karlsson, Magnus" <magnus.karlsson@intel.com>
Subject: Re: XDP socket rings, and LKMM litmus tests
Date: Tue, 2 Mar 2021 12:41:57 -0800 [thread overview]
Message-ID: <20210302204157.GR2696@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <CAJ+HfNj-_P=LpkrUjxcOR73ffMXwsJ+o+zMTfmkiuH2zZ5XCLQ@mail.gmail.com>
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@kernel.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
next prev parent reply other threads:[~2021-03-03 3:56 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-02 18:46 XDP socket rings, and LKMM litmus tests Björn Töpel
2021-03-02 19:57 ` Paul E. McKenney
2021-03-02 20:04 ` Paul E. McKenney
2021-03-02 20:37 ` Björn Töpel
2021-03-02 20:24 ` Björn Töpel
2021-03-02 20:41 ` Paul E. McKenney [this message]
2021-03-02 20:51 ` Björn Töpel
2021-03-02 21:14 ` Alan Stern
2021-03-02 23:50 ` Paul E. McKenney
2021-03-03 6:37 ` maranget
2021-03-03 16:54 ` Paul E. McKenney
2021-03-03 17:12 ` Alan Stern
2021-03-03 17:37 ` maranget
2021-03-03 17:39 ` maranget
2021-03-03 21:56 ` Paul E. McKenney
2021-03-03 19:40 ` Alan Stern
2021-03-03 17:40 ` Paul E. McKenney
2021-03-03 20:22 ` Alan Stern
2021-03-03 22:03 ` Paul E. McKenney
2021-03-04 3:21 ` Alan Stern
2021-03-04 5:04 ` Paul E. McKenney
2021-03-04 15:35 ` Alan Stern
2021-03-04 19:05 ` Paul E. McKenney
2021-03-04 21:27 ` Alan Stern
2021-03-04 22:05 ` Paul E. McKenney
2021-03-04 1:26 ` Boqun Feng
2021-03-04 3:13 ` Alan Stern
2021-03-04 6:33 ` Boqun Feng
2021-03-04 16:11 ` Alan Stern
2021-03-05 1:12 ` Boqun Feng
2021-03-05 16:15 ` Alan Stern
2021-03-04 15:44 ` maranget
2021-03-04 19:07 ` Paul E. McKenney
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