From: Linus Torvalds <torvalds@linux-foundation.org>
To: Mathieu Desnoyers <compudj@krystal.dyndns.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Nick Piggin <nickpiggin@yahoo.com.au>,
Bryan Wu <cooloney@kernel.org>,
linux-kernel@vger.kernel.org, ltt-dev@lists.casi.polymtl.ca,
uclinux-dist-devel@blackfin.uclinux.org
Subject: Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost)
Date: Fri, 13 Feb 2009 08:18:46 -0800 (PST) [thread overview]
Message-ID: <alpine.LFD.2.00.0902130808050.3099@localhost.localdomain> (raw)
In-Reply-To: <20090213155507.GA2838@Krystal>
On Fri, 13 Feb 2009, Mathieu Desnoyers wrote:
>
> I created also
>
> _STORE_SHARED()
> _LOAD_SHARED()
>
> which identify the variables which need to have cache flush done before
> (load) or after (store). So we get both speed and identification when
> needed (if we need to do batch updates linked with a single cache flush).
> e.g.
The thing is, THAT JUST ABSOLUTELY SUCKS.
Lookie here - we don't want to flush the cache at every load of a shared
variable. There's no reason to. If you don't care about the orderign, you
might as well get the old values. That's what memory ordering _means_, for
chissake! In the absense of locks, loads may get stale values. It's that
easy.
A lot of code wants to access multiple variables, and they are potentially
nearby, and in the same cacheline. Making them all use _LOAD_SHARED() adds
absolutely no value - and makes it MUCH MUCH SLOWER.
So what's the answer?
I already outlined it: either you use locks (which will do the magic for
you), or you use memory barriers. In no case do you make the access magic,
unless you have a compiler issue where you are afraid that the compiler
would turn it into _multiple_ accesses and potentially get inconsistent
results.
So the point about ACCESS_ONCE() is not, and never has been, about
re-ordering. We know that the CPU may re-order the accesses and give us
stale values (or values from the "future" wrt the other accesses around
it). That's not the point. The point of ACCESS_ONCE() is that we get
exactly _one_ value, and not two different ones (or none at all) because
of the compiler either re-loading it several times or not re-loading it at
all.
Anybody who confuses ACCESS_ONCE() with ordering is simply confused.
And we don't want to make any "load with cache flush" either. Which side
should the cache flush be on? Before? After? Both? Atomically? There is no
sane semantics for that.
The only remaining sane semantics is to depend on memory barriers, and
then make a magic memory barrier that is extra weak and doesn't order
anythign at all, but just says "syncronize very weakly".
And I think we have that in "cpu_relax()". Because if you have somebody
doing shared memory accesses in a loop without any memory barriers or
locks or anything (ie the _ordering_ doesn't matter, only that some value
has been seen), then dang it, I can't see how you can _possibly_ use
anything else than that "cpu_relax()" somewhere in that loop.
Linus
next prev parent reply other threads:[~2009-02-13 16:19 UTC|newest]
Thread overview: 116+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-02-06 3:05 [RFC git tree] Userspace RCU (urcu) for Linux Mathieu Desnoyers
2009-02-06 4:58 ` [RFC git tree] Userspace RCU (urcu) for Linux (repost) Mathieu Desnoyers
2009-02-06 13:06 ` Paul E. McKenney
2009-02-06 16:34 ` Paul E. McKenney
2009-02-07 15:10 ` Paul E. McKenney
2009-02-07 22:16 ` Paul E. McKenney
2009-02-08 0:19 ` Mathieu Desnoyers
2009-02-07 23:38 ` Mathieu Desnoyers
2009-02-08 0:44 ` Paul E. McKenney
2009-02-08 21:46 ` Mathieu Desnoyers
2009-02-08 22:36 ` Paul E. McKenney
2009-02-09 0:24 ` Paul E. McKenney
2009-02-09 0:54 ` Mathieu Desnoyers
2009-02-09 1:08 ` [ltt-dev] " Mathieu Desnoyers
2009-02-09 3:47 ` Paul E. McKenney
2009-02-09 3:42 ` Paul E. McKenney
2009-02-09 0:40 ` [ltt-dev] " Mathieu Desnoyers
2009-02-08 22:44 ` Mathieu Desnoyers
2009-02-09 4:11 ` Paul E. McKenney
2009-02-09 4:53 ` Mathieu Desnoyers
2009-02-09 5:17 ` [ltt-dev] " Mathieu Desnoyers
2009-02-09 7:03 ` Mathieu Desnoyers
2009-02-09 15:33 ` Paul E. McKenney
2009-02-10 19:17 ` Mathieu Desnoyers
2009-02-10 21:16 ` Paul E. McKenney
2009-02-10 21:28 ` Mathieu Desnoyers
2009-02-10 22:21 ` Paul E. McKenney
2009-02-10 22:58 ` Paul E. McKenney
2009-02-10 23:01 ` Paul E. McKenney
2009-02-11 0:57 ` Mathieu Desnoyers
2009-02-11 5:28 ` Paul E. McKenney
2009-02-11 6:35 ` Mathieu Desnoyers
2009-02-11 15:32 ` Paul E. McKenney
2009-02-11 18:52 ` Mathieu Desnoyers
2009-02-11 20:09 ` Paul E. McKenney
2009-02-11 21:42 ` Mathieu Desnoyers
2009-02-11 22:08 ` Mathieu Desnoyers
[not found] ` <20090212003549.GU6694@linux.vnet.ibm.com>
2009-02-12 2:33 ` Paul E. McKenney
2009-02-12 2:37 ` Paul E. McKenney
2009-02-12 4:10 ` Mathieu Desnoyers
2009-02-12 5:09 ` Paul E. McKenney
2009-02-12 5:47 ` Mathieu Desnoyers
2009-02-12 16:18 ` Paul E. McKenney
2009-02-12 18:40 ` Mathieu Desnoyers
2009-02-12 20:28 ` Paul E. McKenney
2009-02-12 21:27 ` Mathieu Desnoyers
2009-02-12 23:26 ` Paul E. McKenney
2009-02-13 13:12 ` Mathieu Desnoyers
2009-02-12 4:08 ` Mathieu Desnoyers
2009-02-12 5:01 ` Paul E. McKenney
2009-02-12 7:05 ` Mathieu Desnoyers
2009-02-12 16:46 ` Paul E. McKenney
2009-02-12 19:29 ` Mathieu Desnoyers
2009-02-12 20:02 ` Paul E. McKenney
2009-02-12 20:09 ` Mathieu Desnoyers
2009-02-12 20:35 ` Paul E. McKenney
2009-02-12 21:15 ` Mathieu Desnoyers
2009-02-12 20:13 ` Linus Torvalds
2009-02-12 20:39 ` Paul E. McKenney
2009-02-12 21:15 ` Linus Torvalds
2009-02-12 21:59 ` Paul E. McKenney
2009-02-13 13:50 ` Nick Piggin
2009-02-13 14:56 ` Paul E. McKenney
2009-02-13 15:10 ` Mathieu Desnoyers
2009-02-13 15:55 ` Mathieu Desnoyers
2009-02-13 16:18 ` Linus Torvalds [this message]
2009-02-13 17:33 ` Mathieu Desnoyers
2009-02-13 17:53 ` Linus Torvalds
2009-02-13 18:09 ` Linus Torvalds
2009-02-13 18:54 ` Mathieu Desnoyers
2009-02-13 19:36 ` Paul E. McKenney
2009-02-14 5:07 ` Mike Frysinger
2009-02-14 5:20 ` Paul E. McKenney
2009-02-14 5:46 ` Mike Frysinger
2009-02-14 15:06 ` Paul E. McKenney
2009-02-14 17:37 ` Mike Frysinger
2009-02-22 14:23 ` Pavel Machek
2009-02-22 18:28 ` Mike Frysinger
2009-02-14 6:42 ` Mathieu Desnoyers
2009-02-14 3:15 ` [Uclinux-dist-devel] " Mike Frysinger
2009-02-13 18:40 ` Mathieu Desnoyers
2009-02-13 16:05 ` Linus Torvalds
2009-02-14 3:11 ` [Uclinux-dist-devel] " Mike Frysinger
2009-02-14 4:58 ` Robin Getz
2009-02-12 19:38 ` Mathieu Desnoyers
2009-02-12 20:17 ` Paul E. McKenney
2009-02-12 21:53 ` Mathieu Desnoyers
2009-02-12 23:04 ` Paul E. McKenney
2009-02-13 12:49 ` Mathieu Desnoyers
2009-02-11 5:08 ` Lai Jiangshan
2009-02-11 8:58 ` Mathieu Desnoyers
2009-02-09 13:23 ` Paul E. McKenney
2009-02-09 17:28 ` Mathieu Desnoyers
2009-02-09 17:47 ` Paul E. McKenney
2009-02-09 18:13 ` Mathieu Desnoyers
2009-02-09 18:19 ` Mathieu Desnoyers
2009-02-09 18:37 ` Paul E. McKenney
2009-02-09 18:49 ` Paul E. McKenney
2009-02-09 19:05 ` Mathieu Desnoyers
2009-02-09 19:15 ` Mathieu Desnoyers
2009-02-09 19:35 ` Paul E. McKenney
2009-02-09 19:23 ` Paul E. McKenney
2009-02-09 13:16 ` Paul E. McKenney
2009-02-09 17:19 ` Bert Wesarg
2009-02-09 17:34 ` Paul E. McKenney
2009-02-09 17:35 ` Bert Wesarg
2009-02-09 17:40 ` Paul E. McKenney
2009-02-09 17:42 ` Mathieu Desnoyers
2009-02-09 18:00 ` Paul E. McKenney
2009-02-09 17:45 ` Bert Wesarg
2009-02-09 17:59 ` Paul E. McKenney
2009-02-07 22:56 ` Kyle Moffett
2009-02-07 23:50 ` Mathieu Desnoyers
2009-02-08 0:13 ` Paul E. McKenney
2009-02-06 8:55 ` [RFC git tree] Userspace RCU (urcu) for Linux Bert Wesarg
2009-02-06 11:36 ` Mathieu Desnoyers
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