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

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Message-ID: <20200218225455.GN2935@paulmck-ThinkPad-P72>
Date:   Tue, 18 Feb 2020 14:54:55 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.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, Feb 18, 2020 at 05:45:03PM -0500, Steven Rostedt wrote:
> 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."
> 
> ?

That sounds more like something I might put in rcutodo.html as a statement
of the RCU approach to KCSAN reports.

But switching to a different situation (for variety, if nothing else),
what about the commit shown below?

							Thanx, Paul

------------------------------------------------------------------------

commit 35bc02b04a041f32470ae6d959c549bcce8483db
Author: Paul E. McKenney <paulmck@...nel.org>
Date:   Tue Feb 18 13:41:02 2020 -0800

    rcutorture: Mark data-race potential for rcu_barrier() test statistics
    
    The n_barrier_successes, n_barrier_attempts, and
    n_rcu_torture_barrier_error variables are updated (without access
    markings) by the main rcu_barrier() test kthread, and accessed (also
    without access markings) by the rcu_torture_stats() kthread.  This of
    course can result in KCSAN complaints.
    
    Because the accesses are in diagnostic prints, this commit uses
    data_race() to excuse the diagnostic prints from the data race.  If this
    were to ever cause bogus statistics prints (for example, due to store
    tearing), any misleading information would be disambiguated by the
    presence or absence of an rcutorture splat.
    
    This data race was reported by KCSAN.  Not appropriate for backporting
    due to failure being unlikely and due to the mild consequences of the
    failure, namely a confusing rcutorture console message.
    
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 5453bd5..b3301f3 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1444,9 +1444,9 @@ rcu_torture_stats_print(void)
 		atomic_long_read(&n_rcu_torture_timers));
 	torture_onoff_stats();
 	pr_cont("barrier: %ld/%ld:%ld\n",
-		n_barrier_successes,
-		n_barrier_attempts,
-		n_rcu_torture_barrier_error);
+		data_race(n_barrier_successes),
+		data_race(n_barrier_attempts),
+		data_race(n_rcu_torture_barrier_error));
 
 	pr_alert("%s%s ", torture_type, TORTURE_FLAG);
 	if (atomic_read(&n_rcu_torture_mberror) ||