From: "Paul E. McKenney" <paulmck@kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Jonas Oberhauser <jonas.oberhauser@huawei.com>,
Peter Zijlstra <peterz@infradead.org>,
"parri.andrea" <parri.andrea@gmail.com>, will <will@kernel.org>,
"boqun.feng" <boqun.feng@gmail.com>, npiggin <npiggin@gmail.com>,
dhowells <dhowells@redhat.com>, "j.alglave" <j.alglave@ucl.ac.uk>,
"luc.maranget" <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
dlustig <dlustig@nvidia.com>, joel <joel@joelfernandes.org>,
urezki <urezki@gmail.com>,
quic_neeraju <quic_neeraju@quicinc.com>,
frederic <frederic@kernel.org>,
Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)
Date: Thu, 19 Jan 2023 10:43:38 -0800 [thread overview]
Message-ID: <20230119184338.GU2948950@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <Y8lynRI35cFeuqb5@rowland.harvard.edu>
On Thu, Jan 19, 2023 at 11:41:01AM -0500, Alan Stern wrote:
> On Thu, Jan 19, 2023 at 12:22:50PM +0100, Jonas Oberhauser wrote:
> >
> >
> > On 1/19/2023 3:28 AM, Alan Stern wrote:
> > > > This is a permanent error; I've given up. Sorry it didn't
> > > work out.
> >
> > [It seems the e-mail still reached me through the mailing list]
>
> [For everyone else, Jonas is referring to the fact that the last two
> emails I sent to his huaweicloud.com address could not be delivered, so
> I copied them off-list to his huawei.com address.]
>
> > > > I consider that a hack though and don't like it.
> > > It _is_ a bit of a hack, but not a huge one. srcu_read_lock() really
> > > is a lot like a load, in that it returns a value obtained by reading
> > > something from memory (along with some other operations, though, so it
> > > isn't a simple straightforward read -- perhaps more like an
> > > atomic_inc_return_relaxed).
> > The issue I have with this is that it might create accidental ordering. How
> > does it behave when you throw fences in the mix?
>
> I think this isn't going to be a problem. Certainly any real
> implementation of scru_read_lock() is going to involve some actual load
> operations, so any unintentional ordering caused by fences will also
> apply to real executions. Likewise for srcu_read_unlock and store
> operations.
>
> > It really does not work like an increment at all, I think srcu_read_lock()
> > only reads the currently active index, but the index is changed by
> > srcu_sync. But even that is an implementation detail of sorts. I think the
> > best way to think of it would be for srcu_read_lock to just return an
> > arbitrary value.
>
> I think I'll stick to it always returning the initial value. Paul said
> that would be okay.
Just confirming.
> > The user can not rely on any kind of "accidental" rfe edges between these
> > events for ordering.
> >
> > Perhaps if you flag any use of these values in address or control
> > dependencies, as well as any event which depends on more than one of these
> > values, you could prove that it's impossible to contrain the behavior
> > through these rfe(and/or co) edges because you can anyways never inspect the
> > value returned by the operation (except to pass it into srcu_unlock).
> >
> > Or you might be able to explicitly eliminate the events everywhere, just
> > like you have done for carry-dep in your patch.
>
> On second thought, I'll make it impossible to read from the
> srcu_read_unlock events by removing them from the rf (and rfi/rfe)
> relation. Then it won't be necessary to change carry-dep or anything
> else.
Although that works very well for srcu_read_lock() and srcu_read_unlock(),
it would be an issue for srcu_down_read() and srcu_up_read(). But one
thing at a time! ;-)
Thanx, Paul
next prev parent reply other threads:[~2023-01-19 18:43 UTC|newest]
Thread overview: 161+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20220921173109.GA1214281@paulmck-ThinkPad-P17-Gen-1>
[not found] ` <YytfFiMT2Xsdwowf@rowland.harvard.edu>
[not found] ` <YywXuzZ/922LHfjI@hirez.programming.kicks-ass.net>
[not found] ` <114ECED5-FED1-4361-94F7-8D9BC02449B7>
[not found] ` <YzSAnclenTz7KQyt@rowland.harvard.edu>
[not found] ` <f763bd5ff835458d8750b61da47fe316@huawei.com>
2023-01-03 18:56 ` Internal vs. external barriers (was: Re: Interesting LKMM litmus test) Alan Stern
2023-01-04 15:37 ` Andrea Parri
2023-01-04 20:58 ` Alan Stern
[not found] ` <ee186bc17a5e48298a5373f688496dce@huawei.com>
2023-01-05 17:32 ` Paul E. McKenney
[not found] ` <bea712c82e6346f8973399a5711ff78a@huawei.com>
2023-01-11 15:06 ` Alan Stern
[not found] ` <768ffe7edc7f4ddfacd5b0a8e844ed84@huawei.com>
2023-01-11 17:01 ` Alan Stern
[not found] ` <07579baee4b84532a76ea8b0b33052bb@huawei.com>
2023-01-12 21:57 ` Paul E. McKenney
2023-01-13 16:38 ` Alan Stern
2023-01-13 19:54 ` Paul E. McKenney
[not found] ` <06a8aef7eb8d46bca34521a80880dae3@huawei.com>
2023-01-14 17:42 ` Paul E. McKenney
[not found] ` <e51c82a113484b6bb62354a49376f248@huawei.com>
2023-01-14 16:42 ` Alan Stern
2023-01-17 17:48 ` Jonas Oberhauser
2023-01-17 21:19 ` Alan Stern
2023-01-18 11:25 ` Jonas Oberhauser
2023-01-19 2:28 ` Alan Stern
2023-01-19 11:22 ` Jonas Oberhauser
2023-01-19 16:41 ` Alan Stern
2023-01-19 18:43 ` Paul E. McKenney [this message]
2023-01-23 16:16 ` Jonas Oberhauser
2023-01-23 19:58 ` Alan Stern
2023-01-23 20:06 ` Jonas Oberhauser
2023-01-23 20:41 ` Alan Stern
2023-01-24 13:21 ` Jonas Oberhauser
2023-01-24 15:54 ` Jonas Oberhauser
2023-01-24 17:22 ` Alan Stern
[not found] ` <4c1abc7733794519ad7c5153ae8b58f9@huawei.com>
2023-01-13 16:28 ` Alan Stern
2023-01-13 20:07 ` Paul E. McKenney
2023-01-13 20:32 ` Paul E. McKenney
2023-01-14 17:40 ` Alan Stern
2023-01-14 17:48 ` Paul E. McKenney
[not found] ` <136d019d8c8049f6b737627df830e66f@huawei.com>
2023-01-14 17:53 ` Paul E. McKenney
2023-01-14 18:15 ` Paul E. McKenney
2023-01-14 19:58 ` Alan Stern
2023-01-15 5:19 ` Paul E. McKenney
2023-01-14 20:19 ` Alan Stern
2023-01-15 5:15 ` Paul E. McKenney
2023-01-15 16:23 ` Alan Stern
2023-01-15 18:10 ` Paul E. McKenney
2023-01-15 20:46 ` Alan Stern
2023-01-16 4:23 ` Paul E. McKenney
2023-01-16 18:11 ` Alan Stern
2023-01-16 19:06 ` Paul E. McKenney
2023-01-16 19:20 ` Alan Stern
2023-01-16 22:13 ` Paul E. McKenney
2023-01-17 11:46 ` Andrea Parri
2023-01-17 15:14 ` Paul E. McKenney
2023-01-17 15:56 ` Alan Stern
2023-01-17 17:43 ` Paul E. McKenney
2023-01-17 18:27 ` Jonas Oberhauser
2023-01-17 18:55 ` Paul E. McKenney
2023-01-17 20:20 ` Jonas Oberhauser
2023-01-17 20:15 ` Alan Stern
2023-01-18 3:50 ` Paul E. McKenney
2023-01-18 16:50 ` Alan Stern
2023-01-18 19:42 ` Jonas Oberhauser
2023-01-18 20:19 ` Paul E. McKenney
2023-01-18 20:30 ` Jonas Oberhauser
2023-01-18 21:12 ` Paul E. McKenney
2023-01-18 21:24 ` Jonas Oberhauser
2023-01-19 0:11 ` Paul E. McKenney
2023-01-19 13:39 ` Jonas Oberhauser
2023-01-19 18:41 ` Paul E. McKenney
2023-01-19 19:51 ` Alan Stern
2023-01-19 21:53 ` Paul E. McKenney
2023-01-19 22:04 ` Alan Stern
2023-01-19 23:03 ` Paul E. McKenney
2023-01-20 9:43 ` Jonas Oberhauser
2023-01-20 15:39 ` Paul E. McKenney
2023-01-20 20:46 ` Jonas Oberhauser
2023-01-20 21:37 ` Paul E. McKenney
2023-01-20 22:36 ` Jonas Oberhauser
2023-01-20 23:19 ` Paul E. McKenney
2023-01-21 0:03 ` Jonas Oberhauser
2023-01-21 0:34 ` Paul E. McKenney
2023-01-20 3:55 ` Paul E. McKenney
2023-01-20 9:20 ` Jonas Oberhauser
2023-01-20 12:34 ` Jonas Oberhauser
2023-01-20 12:51 ` Jonas Oberhauser
2023-01-20 15:32 ` Paul E. McKenney
2023-01-20 20:56 ` Jonas Oberhauser
2023-01-20 21:40 ` Paul E. McKenney
2023-01-20 16:14 ` Alan Stern
2023-01-20 17:30 ` Paul E. McKenney
2023-01-20 18:15 ` Alan Stern
2023-01-20 18:59 ` Paul E. McKenney
2023-01-20 10:13 ` Jonas Oberhauser
2023-01-20 15:47 ` Paul E. McKenney
2023-01-20 22:21 ` Jonas Oberhauser
2023-01-20 16:18 ` Alan Stern
2023-01-20 21:41 ` Jonas Oberhauser
2023-01-21 4:38 ` Paul E. McKenney
2023-01-21 17:36 ` Alan Stern
2023-01-21 18:40 ` Paul E. McKenney
2023-01-21 19:56 ` Alan Stern
2023-01-21 20:10 ` Paul E. McKenney
2023-01-21 21:03 ` Alan Stern
2023-01-21 23:49 ` Paul E. McKenney
2023-01-23 11:48 ` Jonas Oberhauser
2023-01-23 15:55 ` Alan Stern
2023-01-23 19:40 ` Jonas Oberhauser
2023-01-23 20:34 ` Alan Stern
2023-01-18 20:06 ` Paul E. McKenney
2023-01-18 20:54 ` Alan Stern
2023-01-18 21:05 ` Jonas Oberhauser
2023-01-19 0:02 ` Paul E. McKenney
2023-01-19 2:19 ` Alan Stern
2023-01-19 11:23 ` Paul E. McKenney
2023-01-20 16:01 ` Alan Stern
2023-01-20 17:58 ` Paul E. McKenney
2023-01-20 18:37 ` Alan Stern
2023-01-20 19:20 ` Paul E. McKenney
2023-01-20 20:36 ` Alan Stern
2023-01-20 21:20 ` Paul E. McKenney
2023-01-22 20:32 ` Alan Stern
2023-01-23 20:16 ` Paul E. McKenney
2023-01-24 2:18 ` Alan Stern
2023-01-24 4:06 ` Paul E. McKenney
2023-01-24 11:09 ` Andrea Parri
2023-01-24 14:54 ` Paul E. McKenney
2023-01-24 15:11 ` Jonas Oberhauser
2023-01-24 16:22 ` Paul E. McKenney
2023-01-24 16:39 ` Jonas Oberhauser
2023-01-24 17:26 ` Paul E. McKenney
2023-01-24 19:30 ` Jonas Oberhauser
2023-01-24 22:15 ` Paul E. McKenney
2023-01-24 22:35 ` Alan Stern
2023-01-24 22:54 ` Paul E. McKenney
2023-01-25 1:54 ` Alan Stern
2023-01-25 2:20 ` Paul E. McKenney
2023-01-25 13:10 ` Jonas Oberhauser
2023-01-25 15:05 ` Paul E. McKenney
2023-01-25 15:34 ` Alan Stern
2023-01-25 17:18 ` Paul E. McKenney
2023-01-25 17:42 ` Jonas Oberhauser
2023-01-25 19:08 ` Alan Stern
2023-01-25 19:46 ` Paul E. McKenney
2023-01-25 20:36 ` Andrea Parri
2023-01-25 21:10 ` Jonas Oberhauser
2023-01-25 21:23 ` Paul E. McKenney
2023-01-25 20:46 ` Alan Stern
2023-01-25 21:38 ` Paul E. McKenney
2023-01-25 23:33 ` Paul E. McKenney
2023-01-26 1:45 ` Alan Stern
2023-01-26 1:53 ` Paul E. McKenney
2023-01-26 12:17 ` Jonas Oberhauser
2023-01-26 18:48 ` Paul E. McKenney
2023-01-27 15:03 ` Jonas Oberhauser
2023-01-27 16:50 ` Paul E. McKenney
2023-01-27 16:54 ` Paul E. McKenney
2023-01-18 19:57 ` Jonas Oberhauser
2023-01-18 21:06 ` Paul E. McKenney
2023-01-18 2:15 ` Alan Stern
2023-01-18 5:17 ` Paul E. McKenney
2023-01-18 16:03 ` Alan Stern
2023-01-18 16:59 ` Boqun Feng
2023-01-18 17:08 ` Alan Stern
2023-01-18 17:41 ` Paul E. McKenney
2023-01-19 19:07 ` Paul E. McKenney
2023-01-14 16:55 ` Alan Stern
2023-01-14 17:35 ` Paul E. McKenney
[not found] ` <17078dd97cb6480f9c51e27058af3197@huawei.com>
2023-01-14 17:27 ` Alan Stern
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=20230119184338.GU2948950@paulmck-ThinkPad-P17-Gen-1 \
--to=paulmck@kernel.org \
--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@huawei.com \
--cc=linux-kernel@vger.kernel.org \
--cc=luc.maranget@inria.fr \
--cc=npiggin@gmail.com \
--cc=parri.andrea@gmail.com \
--cc=peterz@infradead.org \
--cc=quic_neeraju@quicinc.com \
--cc=stern@rowland.harvard.edu \
--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).