From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: paulmck <paulmck@linux.ibm.com>,
Valentin Schneider <valentin.schneider@arm.com>,
"Joel Fernandes, Google" <joel@joelfernandes.org>,
Thomas Gleixner <tglx@linutronix.de>,
Alan Stern <stern@rowland.harvard.edu>,
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 11:02:58 -0400 (EDT) [thread overview]
Message-ID: <1065930957.23914.1566054178444.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <CAHk-=whjEq6uEt0o0Ur9Epa7EKVvEFUVJVFJ+heJCv9ehV7pyA@mail.gmail.com>
----- On Aug 17, 2019, at 4:44 AM, Linus Torvalds torvalds@linux-foundation.org wrote:
>
> 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".
If the only states that "x" can take is 1 or 0, then indeed there seems
to be no point in using a WRITE_ONCE() when paired with a READ_ONCE()
other than for documentation purposes.
However, if the state of "x" can be any pointer value, or a reference
count value, then not using "WRITE_ONCE()" to store a constant leaves
the compiler free to perform that store in more than one memory access.
Based on [1], section "Store tearing", there are situations where this
happens on x86 in the wild today when storing 64-bit constants: the
compiler is then free to decide to use two 32-bit immediate store
instructions.
Thanks,
Mathieu
[1] https://lwn.net/Articles/793253/
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2019-08-17 15:03 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
2019-08-17 15:02 ` Mathieu Desnoyers [this message]
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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