Re: [PATCH RFC context_tracking] Make RCU watch ct_kernel_exit_state() warning

[Paul E. McKenney]

On Wed, Feb 05, 2025 at 06:04:41PM +0100, Frederic Weisbecker wrote:
> Le Sat, Feb 01, 2025 at 10:44:02AM -0800, Paul E. McKenney a écrit :
> > The WARN_ON_ONCE() in ct_kernel_exit_state() follows the call to
> > ct_state_inc(), which means that RCU is not watching this WARN_ON_ONCE().
> > This can (and does) result in extraneous lockdep warnings when this
> > WARN_ON_ONCE() triggers.  These extraneous warnings are the opposite
> > of helpful.
> > 
> > Therefore, invert the WARN_ON_ONCE() condition and move it before the
> > call to ct_state_inc().  This does mean that the ct_state_inc() return
> > value can no longer be used in the WARN_ON_ONCE() condition, so discard
> > this return value and instead use a call to rcu_is_watching_curr_cpu().
> > This call is executed only in CONFIG_RCU_EQS_DEBUG=y kernels, so there
> > is no added overhead in production use.
> > 
> > Reported-by: Breno Leitao <leitao@xxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> 
> Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

Thank you!  I will apply this on my next rebase.

							Thanx, Paul

> > diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> > index 938c48952d26..fb5be6e9b423 100644
> > --- a/kernel/context_tracking.c
> > +++ b/kernel/context_tracking.c
> > @@ -80,17 +80,16 @@ static __always_inline void rcu_task_trace_heavyweight_exit(void)
> >   */
> >  static noinstr void ct_kernel_exit_state(int offset)
> >  {
> > -	int seq;
> > -
> >  	/*
> >  	 * CPUs seeing atomic_add_return() must see prior RCU read-side
> >  	 * critical sections, and we also must force ordering with the
> >  	 * next idle sojourn.
> >  	 */
> >  	rcu_task_trace_heavyweight_enter();  // Before CT state update!
> > -	seq = ct_state_inc(offset);
> > -	// RCU is no longer watching.  Better be in extended quiescent state!
> > -	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & CT_RCU_WATCHING));
> > +	// RCU is still watching.  Better not be in extended quiescent state!
> > +	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !rcu_is_watching_curr_cpu());
> > +	(void)ct_state_inc(offset);
> > +	// RCU is no longer watching.
> >  }
> >  
> >  /*