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From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Valentin Schneider <valentin.schneider@arm.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Alan Stern <stern@rowland.harvard.edu>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	rostedt <rostedt@goodmis.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Will Deacon <will.deacon@arm.com>,
	David Howells <dhowells@redhat.com>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates
Date: Sat, 17 Aug 2019 01:44:04 -0700	[thread overview]
Message-ID: <CAHk-=whjEq6uEt0o0Ur9Epa7EKVvEFUVJVFJ+heJCv9ehV7pyA@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wiOhiAJVU71968tAND6rrEJSaYPg7DXK6Y6iiz7_RJACw@mail.gmail.com>

On Sat, Aug 17, 2019 at 1:28 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
>    unsigned int bits = some_global_value;
>    ...test different bits in in 'bits' ...
>
> can easily cause multiple reads (particularly on a CPU that has a
> "test bits in memory" instruction and a lack of registers.
>
> So then doing it as
>
>    unsigned int bits = READ_ONCE(some_global_value);
>    .. test different bits in 'bits'...

Side note: this is likely the best example of actual WRITE_ONCE() use
too: if you have that global value with multiple bits that actually
have some interdependencies, then doing

    some_global_value = some_complex_expression();

might be reasonably compiled to do several rmw instructions to update
'some_global_value'

So then

   WRITE_ONCE(some_global_value, some_complex_expression());

really can be a good thing - it clearly just writes things once, and
it also documents the whole "write one or the other" value, not some
mid-way one, when you then look at the READ_ONCE() thing.

But I'm seeing a lot of WRITE_ONCE(x, constantvalue) kind of things
and don't seem to find a lot of reason to think that they are any
inherently better than "x = constantvalue".

(In contrast, using "smp_store_release(flag, true)" has inherent
value, because it actually implies a memory barrier wrt previous
writes, in ways that WRITE_ONCE() or a direct assignment does not.)

Ok, enough blathering. I think I've made my point.

              Linus

  reply	other threads:[~2019-08-17  8:44 UTC|newest]

Thread overview: 60+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-18 10:29 WARNING in tracepoint_probe_register_prio (3) syzbot
2019-08-16  0:11 ` syzbot
2019-08-16 14:26   ` [PATCH 1/1] Fix: trace sched switch start/stop racy updates Mathieu Desnoyers
2019-08-16 16:25     ` Steven Rostedt
2019-08-16 16:48       ` Valentin Schneider
2019-08-16 17:04         ` Steven Rostedt
2019-08-16 17:41           ` Mathieu Desnoyers
2019-08-16 19:18             ` Steven Rostedt
2019-08-16 19:19             ` Alan Stern
2019-08-16 20:44               ` Joel Fernandes
2019-08-16 20:49                 ` Thomas Gleixner
2019-08-16 20:57                   ` Joel Fernandes
2019-08-16 22:27                     ` Valentin Schneider
2019-08-16 22:57                       ` Linus Torvalds
2019-08-17  1:41                         ` Mathieu Desnoyers
2019-08-17  4:52                         ` Paul E. McKenney
2019-08-17  8:28                           ` Linus Torvalds
2019-08-17  8:44                             ` Linus Torvalds [this message]
2019-08-17 15:02                               ` Mathieu Desnoyers
2019-08-17 20:03                                 ` Valentin Schneider
2019-08-17 23:00                                   ` Paul E. McKenney
2019-08-19 10:34                                     ` Valentin Schneider
2019-08-17 22:28                             ` Paul E. McKenney
2019-08-20 14:01                           ` Peter Zijlstra
2019-08-20 20:31                             ` Paul E. McKenney
2019-08-20 20:39                               ` Peter Zijlstra
2019-08-20 20:52                                 ` Paul E. McKenney
2019-08-16 21:04                   ` Linus Torvalds
2019-08-17  1:36                     ` Mathieu Desnoyers
2019-08-17  2:13                       ` Steven Rostedt
2019-08-17 14:40                         ` Mathieu Desnoyers
2019-08-17 15:26                           ` Steven Rostedt
2019-08-17 15:55                             ` Mathieu Desnoyers
2019-08-17 16:40                               ` Steven Rostedt
2019-08-17 22:06                                 ` Paul E. McKenney
2019-08-17  8:08                       ` Linus Torvalds
2019-08-20 13:56                         ` Peter Zijlstra
2019-08-20 20:29                           ` Paul E. McKenney
2019-08-21 10:32                             ` Will Deacon
2019-08-21 13:23                               ` Paul E. McKenney
2019-08-21 13:32                                 ` Will Deacon
2019-08-21 13:56                                   ` Paul E. McKenney
2019-08-21 16:22                                     ` Will Deacon
2019-08-21 15:33                                 ` Peter Zijlstra
2019-08-21 15:48                                   ` Mathieu Desnoyers
2019-08-21 16:14                                     ` Paul E. McKenney
2019-08-21 19:03                                     ` Joel Fernandes
2019-09-09  6:21                           ` Herbert Xu
2019-08-16 20:49                 ` Steven Rostedt
2019-08-16 20:59                   ` Joel Fernandes
2019-08-17  1:25                   ` Mathieu Desnoyers
2019-08-18  9:15                   ` stable markup was " Pavel Machek
2019-08-16 17:19       ` Mathieu Desnoyers
2019-08-16 19:15         ` Steven Rostedt
2019-08-17 14:27           ` Mathieu Desnoyers
2019-08-17 15:42             ` Steven Rostedt
2019-08-17 15:53               ` Mathieu Desnoyers
2019-08-17 16:43                 ` Steven Rostedt
2019-08-16 12:32 ` WARNING in tracepoint_probe_register_prio (3) syzbot
2019-08-16 12:41   ` Sebastian Andrzej Siewior

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