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From: Alan Stern <stern@rowland.harvard.edu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Paul E. McKenney" <paulmck@kernel.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 10:57:39 -0400	[thread overview]
Message-ID: <20210605145739.GB1712909@rowland.harvard.edu> (raw)
In-Reply-To: <CAHk-=wg0w5L7-iJU_kvEh9stXZoh2srRF4jKToKmSKyHv-njvA@mail.gmail.com>

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.

Alan

  reply	other threads:[~2021-06-05 14:57 UTC|newest]

Thread overview: 121+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-04 10:12 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 [this message]
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
     [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

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