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: Sat, 28 Jan 2023 17:59:52 -0500 [thread overview]
Message-ID: <Y9Wo6OttHC4sUxCS@rowland.harvard.edu> (raw)
In-Reply-To: <Y9WeOTmGCCfjMUtG@andrea>
On Sat, Jan 28, 2023 at 11:14:17PM +0100, Andrea Parri wrote:
> > Evidently the plain-coherence check rules out x=1 at the
> > end, because when I relax that check, x=1 becomes a possible result.
> > Furthermore, the graphical output confirms that this execution has a
> > ww-incoh edge from Wx=2 to Wx=1. But there is no ww-vis edge from Wx=1
> > to Wx=2! How can this be possible? It seems like a bug in herd7.
>
> By default, herd7 performs some edges removal when generating the
> graphical outputs. The option -showraw can be useful to increase
> the "verbosity", for example,
>
> [with "exists (x=2)", output in /tmp/T.dot]
> $ herd7 -conf linux-kernel.cfg T.litmus -show prop -o /tmp -skipchecks plain-coherence -doshow ww-vis -showraw ww-vis
Okay, thanks, that helps a lot.
So here's what we've got. The litmus test:
C hb-and-int
{}
P0(int *x, int *y)
{
*x = 1;
smp_store_release(y, 1);
}
P1(int *x, int *y, int *dx, int *dy, spinlock_t *l)
{
spin_lock(l);
int r1 = READ_ONCE(*dy);
if (r1==1)
spin_unlock(l);
int r0 = smp_load_acquire(y);
if (r0 == 1) {
WRITE_ONCE(*dx,1);
}
}
P2(int *dx, int *dy)
{
WRITE_ONCE(*dy,READ_ONCE(*dx));
}
P3(int *x, spinlock_t *l)
{
spin_lock(l);
smp_mb__after_unlock_lock();
*x = 2;
}
exists (x=2)
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.
This explains why the memory model says there isn't a data race. This
doesn't use the smp_mb__after_unlock_lock at all.
Alan
next prev parent reply other threads:[~2023-01-28 22:59 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 [this message]
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
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=Y9Wo6OttHC4sUxCS@rowland.harvard.edu \
--to=stern@rowland.harvard.edu \
--cc=akiyks@gmail.com \
--cc=boqun.feng@gmail.com \
--cc=dhowells@redhat.com \
--cc=dlustig@nvidia.com \
--cc=frederic@kernel.org \
--cc=j.alglave@ucl.ac.uk \
--cc=joel@joelfernandes.org \
--cc=jonas.oberhauser@huaweicloud.com \
--cc=linux-kernel@vger.kernel.org \
--cc=luc.maranget@inria.fr \
--cc=npiggin@gmail.com \
--cc=parri.andrea@gmail.com \
--cc=paulmck@kernel.org \
--cc=peterz@infradead.org \
--cc=quic_neeraju@quicinc.com \
--cc=urezki@gmail.com \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).