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

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Feb 18, 2020 at 02:54:55PM -0800, Paul E. McKenney wrote:
> 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@kernel.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@kernel.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@kernel.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));

Would it be not worth just fixing the data-race within rcutorture itself?

thanks,

 - Joel