linux-kernel - Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

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Date:   Sat, 17 Aug 2019 12:40:40 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...ux.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

On Sat, 17 Aug 2019 11:55:17 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> ----- On Aug 17, 2019, at 11:26 AM, rostedt rostedt@...dmis.org wrote:
> 
> > On Sat, 17 Aug 2019 10:40:31 -0400 (EDT)
> > Mathieu Desnoyers <mathieu.desnoyers@...icios.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 ;-)

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.

-- Steve