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
next prev parent 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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