From: "Paul E. McKenney" <paulmck@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>,
Peter Zijlstra <peterz@infradead.org>,
Will Deacon <will@kernel.org>,
Andrea Parri <parri.andrea@gmail.com>,
Boqun Feng <boqun.feng@gmail.com>,
Nick Piggin <npiggin@gmail.com>,
David Howells <dhowells@redhat.com>,
Jade Alglave <j.alglave@ucl.ac.uk>,
Luc Maranget <luc.maranget@inria.fr>,
Akira Yokosawa <akiyks@gmail.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-toolchains@vger.kernel.org,
linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()
Date: Fri, 4 Jun 2021 14:40:10 -0700 [thread overview]
Message-ID: <20210604214010.GD4397@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <CAHk-=wgmUbU6XPHz=4NFoLMxH7j_SR-ky4sKzOBrckmvk5AJow@mail.gmail.com>
On Fri, Jun 04, 2021 at 02:27:49PM -0700, Linus Torvalds wrote:
> On Fri, Jun 4, 2021 at 1:56 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > The usual way to prevent it is to use WRITE_ONCE().
>
> The very *documentation example* for "volatile_if()" uses that WRITE_ONCE().
Whew! ;-)
> IOW, the patch that started this discussion has this comment in it:
>
> +/**
> + * volatile_if() - Provide a control-dependency
> + *
> + * volatile_if(READ_ONCE(A))
> + * WRITE_ONCE(B, 1);
> + *
> + * will ensure that the STORE to B happens after the LOAD of A.
>
> and my point is that I don't see *ANY THEORETICALLY POSSIBLE* way that
> that "volatile_if()" could not be just a perfectly regular "if ()".
>
> Can you?
I cannot, maybe due to failure of imagination. But please see below.
> Because we *literally* depend on the fundamental concept of causality
> to make the hardware not re-order those operations.
>
> That is the WHOLE AND ONLY point of this whole construct: we're
> avoiding a possibly expensive hardware barrier operation, because we
> know we have a more fundamental barrier that is INHERENT TO THE
> OPERATION.
>
> And I cannot for the life of me see how a compiler can break that
> fundamental concept of causality either.
>
> Seriously. Tell me how a compiler could _possibly_ turn that into
> something that breaks the fundamental causal relationship. The same
> fundamental causal relationship that is the whole and only reason we
> don't need a memory barrier for the hardware.
>
> And no, there is not a way in hell that the above can be written with
> some kind of semantically visible speculative store without the
> compiler being a total pile of garbage that wouldn't be usable for
> compiling a kernel with.
>
> If your argument is that the compiler can magically insert speculative
> stores that can then be overwritten later, then MY argument is that
> such a compiler could do that for *ANYTHING*. "volatile_if()" wouldn't
> save us.
>
> If that's valid compiler behavior in your opinion, then we have
> exactly two options:
>
> (a) give up
>
> (b) not use that broken garbage of a compiler.
>
> So I can certainly accept the patch with the simpler implementation of
> "volatile_if()", but dammit, I want to see an actual real example
> arguing for why it would be relevant and why the compiler would need
> our help.
>
> Because the EXACT VERY EXAMPLE that was in the patch as-is sure as
> hell is no such thing.
>
> If the intent is to *document* that "this conditional is part of a
> load-conditional-store memory ordering pattern, then that is one
> thing. But if that's the intent, then we might as well just write it
> as
>
> #define volatile_if(x) if (x)
>
> and add a *comment* about why this kind of sequence doesn't need a
> memory barrier.
>
> I'd much rather have that kind of documentation, than have barriers
> that are magical for theoretical compiler issues that aren't real, and
> don't have any grounding in reality.
>
> Without a real and valid example of how this could matter, this is
> just voodoo programming.
>
> We don't actually need to walk three times widdershins around the
> computer before compiling the kernel.That's not how kernel development
> works.
>
> And we don't need to add a "volatile_if()" with magical barriers that
> have no possibility of having real semantic meaning.
>
> So I want to know what the semantic meaning of volatile_if() would be,
> and why it fixes anything that a plain "if()" wouldn't. I want to see
> the sequence where that "volatile_if()" actually *fixes* something.
Here is one use case:
volatile_if(READ_ONCE(A)) {
WRITE_ONCE(B, 1);
do_something();
} else {
WRITE_ONCE(B, 1);
do_something_else();
}
With plain "if", the compiler is within its rights to do this:
tmp = READ_ONCE(A);
WRITE_ONCE(B, 1);
if (tmp)
do_something();
else
do_something_else();
On x86, still no problem. But weaker hardware could now reorder the
store to B before the load from A. With volatile_if(), this reordering
would be prevented.
Thanx, Paul
next prev parent reply other threads:[~2021-06-04 21:40 UTC|newest]
Thread overview: 128+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-04 10:12 [RFC] LKMM: Add volatile_if() Peter Zijlstra
2021-06-04 10:44 ` Will Deacon
2021-06-04 11:13 ` Will Deacon
2021-06-04 11:31 ` Peter Zijlstra
2021-06-04 13:44 ` Will Deacon
2021-06-04 13:56 ` Peter Zijlstra
2021-06-04 15:13 ` Will Deacon
2021-06-04 15:22 ` Peter Zijlstra
2021-06-04 15:36 ` Alan Stern
2021-06-04 15:42 ` Peter Zijlstra
2021-06-04 15:51 ` Alan Stern
2021-06-04 16:17 ` Peter Zijlstra
2021-06-04 18:27 ` Alan Stern
2021-06-04 19:09 ` Linus Torvalds
2021-06-04 19:18 ` Linus Torvalds
2021-06-04 20:56 ` Paul E. McKenney
2021-06-04 21:27 ` Linus Torvalds
2021-06-04 21:40 ` Paul E. McKenney [this message]
2021-06-04 22:19 ` Linus Torvalds
2021-06-05 14:57 ` Alan Stern
2021-06-06 0:14 ` Paul E. McKenney
2021-06-06 1:29 ` Alan Stern
2021-06-06 3:41 ` Linus Torvalds
2021-06-06 4:43 ` Paul E. McKenney
2021-06-06 13:17 ` Segher Boessenkool
2021-06-06 19:07 ` Paul E. McKenney
2021-06-06 12:59 ` Segher Boessenkool
2021-06-06 13:47 ` Alan Stern
2021-06-06 17:13 ` Segher Boessenkool
2021-06-06 18:25 ` Linus Torvalds
2021-06-06 19:19 ` Segher Boessenkool
2021-06-06 18:41 ` Alan Stern
2021-06-06 18:59 ` Jakub Jelinek
2021-06-06 19:15 ` Paul E. McKenney
2021-06-06 19:22 ` Linus Torvalds
2021-06-06 20:11 ` Segher Boessenkool
2021-06-06 21:19 ` Alexander Monakov
2021-06-06 22:38 ` Linus Torvalds
2021-06-06 23:39 ` Rasmus Villemoes
2021-06-06 23:44 ` Rasmus Villemoes
2021-06-07 8:01 ` Alexander Monakov
2021-06-07 8:27 ` Marco Elver
2021-06-07 15:28 ` Paul E. McKenney
2021-06-07 17:04 ` Marco Elver
2021-06-08 9:30 ` Marco Elver
2021-06-08 11:22 ` Peter Zijlstra
2021-06-08 15:28 ` Segher Boessenkool
2021-06-09 12:44 ` Marco Elver
2021-06-09 15:31 ` Segher Boessenkool
2021-06-09 16:13 ` Marco Elver
2021-06-09 17:14 ` Segher Boessenkool
2021-06-09 17:31 ` Nick Desaulniers
2021-06-09 20:24 ` Segher Boessenkool
2021-06-09 18:25 ` Linus Torvalds
2021-06-07 17:52 ` Segher Boessenkool
2021-06-07 18:07 ` Alexander Monakov
2021-06-07 18:18 ` Segher Boessenkool
2021-06-07 17:42 ` Segher Boessenkool
2021-06-07 20:31 ` Linus Torvalds
2021-06-07 22:54 ` Segher Boessenkool
2021-06-06 11:53 ` Segher Boessenkool
2021-06-06 13:45 ` Alan Stern
2021-06-06 18:04 ` Linus Torvalds
2021-06-06 18:22 ` Alan Stern
2021-06-06 18:43 ` Linus Torvalds
2021-06-07 10:43 ` Peter Zijlstra
2021-06-07 11:52 ` Will Deacon
2021-06-07 15:25 ` Paul E. McKenney
2021-06-07 16:02 ` Will Deacon
2021-06-07 18:08 ` Paul E. McKenney
2021-07-30 17:20 ` Jade Alglave
2021-07-30 20:35 ` Alan Stern
2021-08-02 21:18 ` Alan Stern
2021-08-02 23:31 ` Paul E. McKenney
2021-08-04 20:09 ` Alan Stern
2021-08-05 19:47 ` Alan Stern
2021-08-07 0:51 ` Alan Stern
2021-06-06 18:40 ` Segher Boessenkool
2021-06-06 18:48 ` Linus Torvalds
2021-06-06 18:53 ` Linus Torvalds
2021-06-06 19:52 ` Segher Boessenkool
2021-06-06 20:11 ` Linus Torvalds
2021-06-06 20:26 ` Segher Boessenkool
2021-06-06 23:37 ` Paul E. McKenney
2021-06-07 14:12 ` Segher Boessenkool
2021-06-07 15:27 ` Paul E. McKenney
2021-06-07 18:23 ` Segher Boessenkool
2021-06-07 19:51 ` Alan Stern
2021-06-07 20:16 ` Paul E. McKenney
2021-06-07 22:40 ` Segher Boessenkool
2021-06-07 23:26 ` Paul E. McKenney
2021-06-07 10:52 ` Peter Zijlstra
2021-06-07 14:16 ` Segher Boessenkool
2021-06-04 22:05 ` Peter Zijlstra
2021-06-05 3:14 ` Alan Stern
2021-06-05 16:24 ` Linus Torvalds
2021-06-04 15:50 ` Segher Boessenkool
2021-06-04 15:47 ` Segher Boessenkool
2021-06-04 11:44 ` Peter Zijlstra
2021-06-04 14:13 ` Paul E. McKenney
2021-06-04 15:35 ` Segher Boessenkool
2021-06-04 16:10 ` Peter Zijlstra
2021-06-04 16:40 ` Segher Boessenkool
2021-06-04 18:55 ` Paul E. McKenney
2021-06-04 19:53 ` Segher Boessenkool
2021-06-04 20:40 ` Paul E. McKenney
2021-06-06 11:36 ` Segher Boessenkool
2021-06-06 19:01 ` Paul E. McKenney
2021-06-04 14:25 ` Alan Stern
2021-06-04 16:09 ` Segher Boessenkool
2021-06-04 16:33 ` Peter Zijlstra
2021-06-04 16:30 ` Linus Torvalds
2021-06-04 16:37 ` Peter Zijlstra
2021-06-04 16:52 ` Segher Boessenkool
2021-06-04 17:10 ` Linus Torvalds
2021-06-04 17:24 ` Segher Boessenkool
2021-06-04 17:38 ` Linus Torvalds
2021-06-04 18:25 ` Segher Boessenkool
2021-06-04 19:17 ` Peter Zijlstra
2021-06-04 20:43 ` Paul E. McKenney
2021-06-04 18:23 ` Alan Stern
2021-06-08 12:48 ` David Laight
2021-09-24 18:38 ` Mathieu Desnoyers
2021-09-24 19:52 ` Alan Stern
2021-09-24 20:22 ` Mathieu Desnoyers
2021-09-24 19:55 ` Segher Boessenkool
2021-09-24 20:39 ` Mathieu Desnoyers
2021-09-24 22:07 ` Mathieu Desnoyers
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