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

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Message-ID: <20200218201142.GF11457@worktop.programming.kicks-ass.net>
Date:   Tue, 18 Feb 2020 21:11:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     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, rostedt@...dmis.org, 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, 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.