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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Joel Fernandes <joel@joelfernandes.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	rostedt <rostedt@goodmis.org>,
	Valentin Schneider <valentin.schneider@arm.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	paulmck <paulmck@linux.ibm.com>,
	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: Fri, 16 Aug 2019 14:04:56 -0700	[thread overview]
Message-ID: <CAHk-=wh9qDFfWJscAQw_w+obDmZvcE5jWJRdYPKYP6YhgoGgGA@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1908162245440.1923@nanos.tec.linutronix.de>

On Fri, Aug 16, 2019 at 1:49 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Can we finally put a foot down and tell compiler and standard committee
> people to stop this insanity?

It's already effectively done.

Yes, values can be read from memory multiple times if they need
reloading. So "READ_ONCE()" when the value can change is a damn good
idea.

But it should only be used if the value *can* change. Inside a locked
region it is actively pointless and misleading.

Similarly, WRITE_ONCE() should only be used if you have a _reason_ for
using it (notably if you're not holding a lock).

If people use READ_ONCE/WRITE_ONCE when there are locks that prevent
the values from changing, they are only making the code illegible.
Don't do it.

But in the *absence* of locking, READ_ONCE/WRITE_ONCE is usually a
good thing.  The READ_ONCE actually tends to matter, because even if
the value is used only once at a source level, the compiler *could*
decide to do something else.

The WRITE_ONCE() may or may not matter (afaik, thanks to concurrency,
modern C standard does not allow optimistic writes anyway, and we
wouldn't really accept such a compiler option if it did).

But if the write is done without locking, it's good practice just to
show you are aware of the whole "done without locking" part.

              Linus

  parent reply	other threads:[~2019-08-16 21:05 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
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 [this message]
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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