linux-kernel - Re: [PATCH tip/core/rcu 1/3] rcu-tasks: *_ONCE() for rcu_tasks_cbs

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Message-ID: <20200218174503.3d4e4750@gandalf.local.home>
Date:   Tue, 18 Feb 2020 17:45:03 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, dhowells@...hat.com,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
        joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 1/3] rcu-tasks: *_ONCE() for
 rcu_tasks_cbs_head

On Tue, 18 Feb 2020 12:22:26 -0800
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> On Tue, Feb 18, 2020 at 09:11:42PM +0100, Peter Zijlstra wrote:
> > On Tue, Feb 18, 2020 at 08:27:19AM -0800, Paul E. McKenney wrote:  
> > > On Tue, Feb 18, 2020 at 08:56:48AM +0100, Peter Zijlstra wrote:  
> >   
> > > > I just took offence at the Changelog wording. It seems to suggest there
> > > > actually is a problem, there is not.  
> > > 
> > > Quoting the changelog: "Not appropriate for backporting due to failure
> > > being unlikely."  
> > 
> > That implies there is failure, however unlikely.
> > 
> > In this particular case there is absolutely no failure, except perhaps
> > in KCSAN. This patch is a pure annotation such that KCSAN can understand
> > the code.
> > 
> > Like said, I don't object to the actual patch, but I do think it is
> > important to call out false negatives or to describe the actual problem
> > found.  
> 
> I don't feel at all comfortable declaring that there is absolutely
> no possibility of failure.

Perhaps wording it like so:

"There's know known issue with the current code, but the *_ONCE()
annotations here makes KCSAN happy, allowing us to focus on KCSAN
warnings that can help bring about known issues in other code that we
can fix, without being distracted by KCSAN warnings that we do not see
a problem with."

?

-- Steve