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From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Joel Fernandes, Google" <joel@joelfernandes.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	Valentin Schneider <valentin.schneider@arm.com>,
	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 15:06:27 -0700	[thread overview]
Message-ID: <20190817220627.GI28441@linux.ibm.com> (raw)
In-Reply-To: <20190817124040.34c38e19@oasis.local.home>

On Sat, Aug 17, 2019 at 12:40:40PM -0400, Steven Rostedt wrote:
> On Sat, 17 Aug 2019 11:55:17 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
> > ----- On Aug 17, 2019, at 11:26 AM, rostedt rostedt@goodmis.org wrote:
> > 
> > > On Sat, 17 Aug 2019 10:40:31 -0400 (EDT)
> > > Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> > >   
> > >> > I'm now even more against adding the READ_ONCE() or WRITE_ONCE().  
> > >> 
> > >> I'm not convinced by your arguments.  
> > > 
> > > Prove to me that there's an issue here beyond theoretical analysis,
> > > then I'll consider that patch.
> > > 
> > > Show me a compiler used to compile the kernel that zeros out the
> > > increment. Show me were the race actually occurs.
> > > 
> > > I think the READ/WRITE_ONCE() is more confusing than helpful. And
> > > unneeded churn to the code. And really not needed for something that's
> > > not critical to execution.  
> > 
> > I'll have to let the authors of the LWN article speak up on this, because
> > I have limited time to replicate this investigation myself.
> 
> I'll let Paul McKenney convince me then, if he has any spare cycles ;-)

You guys do manage to time these things sometimes.  ;-)

> The one instance in that article is from a 2013 bug, which talks about
> storing a 64 bit value on a 32 bit machine. But the ref count is an int
> (32 bit), and I highly doubt any compiler will split it into 16 bit
> stores for a simple increment. And I don't believe Linux even supports
> any architecture that requires 16 bit stores anymore.

For a machine-sized and aligned increment, it is indeed hard to imagine,
even for me.  I would be more worried about stores of constants with
lots of zero bits between non-zero bits on systems with small-sized
store-immediate instructions.

							Thanx, Paul


  reply	other threads:[~2019-08-17 22:07 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
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 [this message]
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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