From: "Paul E. McKenney" <paulmck@kernel.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
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: Sat, 5 Jun 2021 17:14:18 -0700 [thread overview]
Message-ID: <20210606001418.GH4397@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20210605145739.GB1712909@rowland.harvard.edu>
On Sat, Jun 05, 2021 at 10:57:39AM -0400, Alan Stern wrote:
> On Fri, Jun 04, 2021 at 03:19:11PM -0700, Linus Torvalds wrote:
> > Now, part of this is that I do think that in *general* we should never
> > use this very suble load-cond-store pattern to begin with. We should
> > strive to use more smp_load_acquire() and smp_store_release() if we
> > care about ordering of accesses. They are typically cheap enough, and
> > if there's much of an ordering issue, they are the right things to do.
> >
> > I think the whole "load-to-store ordering" subtle non-ordered case is
> > for very very special cases, when you literally don't have a general
> > memory ordering, you just have an ordering for *one* very particular
> > access. Like some of the very magical code in the rw-semaphore case,
> > or that smp_cond_load_acquire().
> >
> > IOW, I would expect that we have a handful of uses of this thing. And
> > none of them have that "the conditional store is the same on both
> > sides" pattern, afaik.
> >
> > And immediately when the conditional store is different, you end up
> > having a dependency on it that orders it.
> >
> > But I guess I can accept the above made-up example as an "argument",
> > even though I feel it is entirely irrelevant to the actual issues and
> > uses we have.
>
> Indeed, the expansion of the currently proposed version of
>
> volatile_if (A) {
> B;
> } else {
> C;
> }
>
> is basically the same as
>
> if (A) {
> barrier();
> B;
> } else {
> barrier();
> C;
> }
>
> which is just about as easy to write by hand. (For some reason my
> fingers don't like typing "volatile_"; the letters tend to get
> scrambled.)
>
> So given that:
>
> 1. Reliance on control dependencies is uncommon in the kernel,
> and
>
> 2. The loads in A could just be replaced with load_acquires
> at a low penalty (or store-releases could go into B and C),
>
> it seems that we may not need volatile_if at all! The only real reason
> for having it in the first place was to avoid the penalty of
> load-acquire on architectures where it has a significant cost, when the
> control dependency would provide the necessary ordering for free. Such
> architectures are getting less and less common.
That does sound good, but...
Current compilers beg to differ at -O2: https://godbolt.org/z/5K55Gardn
------------------------------------------------------------------------
#define READ_ONCE(x) (*(volatile typeof(x) *)&(x))
#define WRITE_ONCE(x, val) (READ_ONCE(x) = (val))
#define barrier() __asm__ __volatile__("": : :"memory")
int x, y;
int main(int argc, char *argv[])
{
if (READ_ONCE(x)) {
barrier();
WRITE_ONCE(y, 1);
} else {
barrier();
WRITE_ONCE(y, 1);
}
return 0;
}
------------------------------------------------------------------------
Both gcc and clang generate a load followed by a store, with no branch.
ARM gets the same results from both compilers.
As Linus suggested, removing one (but not both!) invocations of barrier()
does cause a branch to be emitted, so maybe that is a way forward.
Assuming it is more than just dumb luck, anyway. :-/
Thanx, Paul
next prev parent reply other threads:[~2021-06-06 0:14 UTC|newest]
Thread overview: 127+ 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
2021-06-04 22:19 ` Linus Torvalds
2021-06-05 14:57 ` Alan Stern
2021-06-06 0:14 ` Paul E. McKenney [this message]
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
[not found] ` <20210730172020.GA32396@knuckles.cs.ucl.ac.uk>
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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