From: Alan Stern <stern@rowland.harvard.edu>
To: Andrea Parri <parri.andrea@gmail.com>
Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
paulmck@kernel.org, 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
Date: Sun, 29 Jan 2023 17:10:49 -0500 [thread overview]
Message-ID: <Y9bu6RMCSsI+wDYf@rowland.harvard.edu> (raw)
In-Reply-To: <Y9aoA2UxvWyNwOtx@andrea>
On Sun, Jan 29, 2023 at 06:11:12PM +0100, Andrea Parri wrote:
> > The reason why Wx=1 ->ww-vis Wx=2:
> >
> > 0:Wx=1 ->po-rel 0:Wy=1 and po-rel < fence < ww-post-bounded.
> >
> > 0:Wy=1 ->rfe 1:Ry=1 ->(hb* & int) 1:Rdy=1 and
> > (rfe ; hb* & int) <= (rfe ; xbstar & int) <= vis.
> >
> > 1:Rdy=1 ->po 1:unlock ->rfe 3:lock ->po 3:Wx=2
> > so 1:Rdy=1 ->po-unlock-lock-po 3:Wx=2
> > and po-unlock-lock-po <= mb <= fence <= w-pre-bounded.
> >
> > Finally, w-post-bounded ; vis ; w-pre-bounded <= ww-vis.
>
> To clarify, po-unlock-lock-po is not a subrelation of mb; see what
> happens without the smp_mb__after_unlock_lock().
Ah, thank you again. That was what I got wrong, and it explains why the
data race appears with Jonas's patch.
This also points out an important unstated fact: All of the inter-CPU
extended fences in the memory model are A-cumulative. In this case the
fence links Rdy=1 on P1 to Wx=3 on P3. We know that 0:Wx=1 is visible
to P1 before the Rdy executes, but what we need is that it is visible to
P3 before Wx=3 executes. The fact that the extended fence is strong
(and therefore A-cumulative) provides this extra guarantee.
Alan
next prev parent reply other threads:[~2023-01-29 22:10 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-26 13:46 [PATCH v2 0/2] Streamlining treatment of smp_mb__after_unlock_lock Jonas Oberhauser
2023-01-26 13:46 ` [PATCH v2 1/2] tools/memory-model: Unify UNLOCK+LOCK pairings to po-unlock-lock-po Jonas Oberhauser
2023-01-26 16:36 ` Alan Stern
2023-01-26 20:08 ` Paul E. McKenney
2023-01-26 23:21 ` Paul E. McKenney
2023-01-27 13:18 ` Jonas Oberhauser
2023-01-27 15:13 ` Paul E. McKenney
2023-01-27 15:57 ` Jonas Oberhauser
2023-01-27 16:48 ` Paul E. McKenney
2023-01-26 13:46 ` [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po Jonas Oberhauser
2023-01-26 16:36 ` Alan Stern
2023-01-27 14:31 ` Jonas Oberhauser
2023-01-28 19:56 ` Alan Stern
2023-01-28 22:14 ` Andrea Parri
2023-01-28 22:21 ` Andrea Parri
2023-01-28 22:59 ` Alan Stern
2023-01-29 5:17 ` Paul E. McKenney
2023-01-29 16:03 ` Alan Stern
2023-01-29 16:21 ` Paul E. McKenney
2023-01-29 17:28 ` Andrea Parri
2023-01-29 18:44 ` Paul E. McKenney
2023-01-29 21:43 ` Boqun Feng
2023-01-29 23:09 ` Paul E. McKenney
2023-01-30 2:18 ` Alan Stern
2023-01-30 4:43 ` Paul E. McKenney
2023-01-29 19:17 ` Paul E. McKenney
2023-01-29 17:11 ` Andrea Parri
2023-01-29 22:10 ` Alan Stern [this message]
2023-01-29 22:19 ` Jonas Oberhauser
2023-01-30 2:39 ` Alan Stern
2023-01-30 4:36 ` Paul E. McKenney
2023-01-30 16:47 ` Alan Stern
2023-01-30 16:50 ` Paul E. McKenney
2023-01-31 13:56 ` Jonas Oberhauser
2023-01-31 15:06 ` Alan Stern
2023-01-31 15:33 ` Jonas Oberhauser
2023-01-31 16:55 ` Alan Stern
2023-02-01 10:37 ` Jonas Oberhauser
2023-01-30 4:46 ` Paul E. McKenney
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