From: Peter Zijlstra <peterz@infradead•org>
To: Linus Torvalds <torvalds@linux-foundation•org>
Cc: Michael Neuling <mikey@neuling•org>,
Mathieu Desnoyers <mathieu.desnoyers@polymtl•ca>,
Oleg Nesterov <oleg@redhat•com>,
LKML <linux-kernel@vger•kernel.org>,
Linux PPC dev <linuxppc-dev@ozlabs•org>,
Anton Blanchard <anton@samba•org>,
Frederic Weisbecker <fweisbec@gmail•com>,
Victor Kaplansky <VICTORK@il•ibm.com>,
"Paul E. McKenney" <paulmck@linux•vnet.ibm.com>
Subject: Re: [RFC] arch: Introduce new TSO memory barrier smp_tmb()
Date: Sun, 3 Nov 2013 21:01:24 +0100 [thread overview]
Message-ID: <20131103200124.GK19466@laptop.lan> (raw)
In-Reply-To: <CA+55aFx_kuvR-0dwJtjvnhtha5QBc5XcLZPRH=WKT+hYVAKOrw@mail.gmail.com>
On Sun, Nov 03, 2013 at 10:08:14AM -0800, Linus Torvalds wrote:
> On Sun, Nov 3, 2013 at 7:17 AM, Peter Zijlstra <peterz@infradead•org> wrote:
> > On Sun, Nov 03, 2013 at 06:40:17AM -0800, Paul E. McKenney wrote:
> >> If there was an smp_tmb(), I would likely use it in rcu_assign_pointer().
> >
> > Well, I'm obviously all for introducing this new barrier, for it will
> > reduce a full mfence on x86 to a compiler barrier. And ppc can use
> > lwsync as opposed to sync afaict. Not sure ARM can do better.
> >
> > ---
> > Subject: arch: Introduce new TSO memory barrier smp_tmb()
>
> This is specialized enough that I would *really* like the name to be
> more descriptive. Compare to the special "smp_read_barrier_depends()"
> maco: it's unusual, and it has very specific semantics, so it gets a
> long and descriptive name.
>
> Memory ordering is subtle enough without then using names that are
> subtle in themselves. mb/rmb/wmb are conceptually pretty simple
> operations, and very basic when talking about memory ordering.
> "acquire" and "release" are less simple, but have descriptive names
> and have very specific uses in locking.
>
> In contrast "smp_tmb()" is a *horrible* name, because TSO is a
> description of the memory ordering, not of a particular barrier. It's
> also not even clear that you can have a "tso barrier", since the
> ordering (like acquire/release) presumably is really about one
> particular *store*, not about some kind of barrier between different
> operations.
>
> So please describe exactly what the semantics that barrier has, and
> then name the barrier that way.
>
> I assume that in this particular case, the semantics RCU wants is
> "write barrier, and no preceding reads can move past this point".
>
> Calling that "smp_tmb()" is f*cking insane, imnsho.
Fair enough; from what I could gather the proposed semantics are
RELEASE+WMB, such that neither reads not writes can cross over, writes
can't cross back, but reads could.
Since both RELEASE and WMB are trivial under TSO the entire thing
collapses.
Now I'm currently completely confused as to what C/C++ wrecks vs actual
proper memory order issues; let alone fully comprehend the case that
started all this.
next prev parent reply other threads:[~2013-11-03 20:01 UTC|newest]
Thread overview: 96+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-22 23:54 perf events ring buffer memory barrier on powerpc Michael Neuling
2013-10-23 7:39 ` Victor Kaplansky
2013-10-23 14:19 ` Frederic Weisbecker
2013-10-23 14:25 ` Frederic Weisbecker
2013-10-25 17:37 ` Peter Zijlstra
2013-10-25 20:31 ` Michael Neuling
2013-10-27 9:00 ` Victor Kaplansky
2013-10-28 9:22 ` Peter Zijlstra
2013-10-28 10:02 ` Frederic Weisbecker
2013-10-28 12:38 ` Victor Kaplansky
2013-10-28 13:26 ` Peter Zijlstra
2013-10-28 16:34 ` Paul E. McKenney
2013-10-28 20:17 ` Oleg Nesterov
2013-10-28 20:58 ` Victor Kaplansky
2013-10-29 10:21 ` Peter Zijlstra
2013-10-29 10:30 ` Peter Zijlstra
2013-10-29 10:35 ` Peter Zijlstra
2013-10-29 20:15 ` Oleg Nesterov
2013-10-29 19:27 ` Vince Weaver
2013-10-30 10:42 ` Peter Zijlstra
2013-10-30 11:48 ` James Hogan
2013-10-30 12:48 ` Peter Zijlstra
2013-10-29 21:23 ` Michael Neuling
2013-10-30 9:27 ` Paul E. McKenney
2013-10-30 11:25 ` Peter Zijlstra
2013-10-30 14:52 ` Victor Kaplansky
2013-10-30 15:39 ` Peter Zijlstra
2013-10-30 17:14 ` Victor Kaplansky
2013-10-30 17:44 ` Peter Zijlstra
2013-10-31 6:16 ` Paul E. McKenney
2013-11-01 13:12 ` Victor Kaplansky
2013-11-02 16:36 ` Paul E. McKenney
2013-11-02 17:26 ` Paul E. McKenney
2013-10-31 6:40 ` Paul E. McKenney
2013-11-01 14:25 ` Victor Kaplansky
2013-11-02 17:28 ` Paul E. McKenney
2013-11-01 14:56 ` Peter Zijlstra
2013-11-02 17:32 ` Paul E. McKenney
2013-11-03 14:40 ` Paul E. McKenney
2013-11-03 15:17 ` [RFC] arch: Introduce new TSO memory barrier smp_tmb() Peter Zijlstra
2013-11-03 18:08 ` Linus Torvalds
2013-11-03 20:01 ` Peter Zijlstra [this message]
2013-11-03 22:42 ` Paul E. McKenney
2013-11-03 23:34 ` Linus Torvalds
2013-11-04 10:51 ` Paul E. McKenney
2013-11-04 11:22 ` Peter Zijlstra
2013-11-04 16:27 ` Paul E. McKenney
2013-11-04 16:48 ` Peter Zijlstra
2013-11-04 19:11 ` Peter Zijlstra
2013-11-04 19:18 ` Peter Zijlstra
2013-11-04 20:54 ` Paul E. McKenney
2013-11-04 20:53 ` Paul E. McKenney
2013-11-05 14:05 ` Will Deacon
2013-11-05 14:49 ` Paul E. McKenney
2013-11-05 18:49 ` Peter Zijlstra
2013-11-06 11:00 ` Will Deacon
2013-11-06 12:39 ` Peter Zijlstra
2013-11-06 12:51 ` Geert Uytterhoeven
2013-11-06 13:57 ` Peter Zijlstra
2013-11-06 18:48 ` Paul E. McKenney
2013-11-06 19:42 ` Peter Zijlstra
2013-11-07 11:17 ` Will Deacon
2013-11-07 13:36 ` Peter Zijlstra
2013-11-07 23:50 ` Mathieu Desnoyers
2013-11-04 11:05 ` Will Deacon
2013-11-04 16:34 ` Paul E. McKenney
2013-11-03 20:59 ` Benjamin Herrenschmidt
2013-11-03 22:43 ` Paul E. McKenney
2013-11-01 16:11 ` perf events ring buffer memory barrier on powerpc Peter Zijlstra
2013-11-02 17:46 ` Paul E. McKenney
2013-11-01 16:18 ` Peter Zijlstra
2013-11-02 17:49 ` Paul E. McKenney
2013-10-30 13:28 ` Victor Kaplansky
2013-10-30 15:51 ` Peter Zijlstra
2013-10-30 18:29 ` Peter Zijlstra
2013-10-30 19:11 ` Peter Zijlstra
2013-10-31 4:33 ` Paul E. McKenney
2013-10-31 4:32 ` Paul E. McKenney
2013-10-31 9:04 ` Peter Zijlstra
2013-10-31 15:07 ` Paul E. McKenney
2013-10-31 15:19 ` Peter Zijlstra
2013-11-01 9:28 ` Paul E. McKenney
2013-11-01 10:30 ` Peter Zijlstra
2013-11-02 15:20 ` Paul E. McKenney
2013-11-04 9:07 ` Peter Zijlstra
2013-11-04 10:00 ` Paul E. McKenney
2013-10-31 9:59 ` Victor Kaplansky
2013-10-31 12:28 ` David Laight
2013-10-31 12:55 ` Victor Kaplansky
2013-10-31 15:25 ` Paul E. McKenney
2013-11-01 16:06 ` Victor Kaplansky
2013-11-01 16:25 ` David Laight
2013-11-01 16:30 ` Victor Kaplansky
2013-11-03 20:57 ` Benjamin Herrenschmidt
2013-11-02 15:46 ` Paul E. McKenney
2013-10-28 19:09 ` Oleg Nesterov
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=20131103200124.GK19466@laptop.lan \
--to=peterz@infradead$(echo .)org \
--cc=VICTORK@il$(echo .)ibm.com \
--cc=anton@samba$(echo .)org \
--cc=fweisbec@gmail$(echo .)com \
--cc=linux-kernel@vger$(echo .)kernel.org \
--cc=linuxppc-dev@ozlabs$(echo .)org \
--cc=mathieu.desnoyers@polymtl$(echo .)ca \
--cc=mikey@neuling$(echo .)org \
--cc=oleg@redhat$(echo .)com \
--cc=paulmck@linux$(echo .)vnet.ibm.com \
--cc=torvalds@linux-foundation$(echo .)org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
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