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Date:   Wed, 19 Jul 2023 13:38:30 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@meta.com, rostedt@goodmis.org,
        Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [PATCH rcu 2/6] rcu: Clarify rcu_is_watching() kernel-doc comment
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On Tue, Jul 18, 2023 at 09:56:38PM -0400, Joel Fernandes wrote:
> Hi Paul,
> 
> On 7/18/23 14:12, Paul E. McKenney wrote:
> > On Tue, Jul 18, 2023 at 08:52:30AM -0400, Joel Fernandes wrote:
> > > Hi Paul,
> > > 
> > > On 7/17/23 14:03, Paul E. McKenney wrote:
> > > > Make it clear that this function always returns either true or false
> > > > without other planned failure modes.
> > > > 
> > > > Reported-by: Masami Hiramatsu <mhiramat@kernel.org>
> > > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > > ---
> > > >    kernel/rcu/tree.c | 12 ++++++++----
> > > >    1 file changed, 8 insertions(+), 4 deletions(-)
> > > > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index 1449cb69a0e0..fae9b4e29c93 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -679,10 +679,14 @@ static void rcu_disable_urgency_upon_qs(struct rcu_data *rdp)
> > > >    /**
> > > >     * rcu_is_watching - see if RCU thinks that the current CPU is not idle
> > > 
> > > Would it be better to modify the 'not idle' to 'not idle from an RCU
> > > viewpoint'? This matches the comments in ct_nmi_enter() as well.
> > 
> > We have the "if RCU thinks that" earlier.
> > 
> > But maybe something like this?
> > 
> >   * rcu_is_watching - RCU read-side critical sections permitted on current CPU?
> > 
> 
> Yes, that's better.
> 
> > > >     *
> > > > - * Return true if RCU is watching the running CPU, which means that this
> > > > - * CPU can safely enter RCU read-side critical sections.  In other words,
> > > > - * if the current CPU is not in its idle loop or is in an interrupt or
> > > > - * NMI handler, return true.
> > > > + * Return @true if RCU is watching the running CPU and @false otherwise.
> > > > + * An @true return means that this CPU can safely enter RCU read-side
> > > > + * critical sections.
> > > > + *
> > > > + * More specifically, if the current CPU is not deep within its idle
> > > > + * loop, return @true.  Note that rcu_is_watching() will return @true if
> > > > + * invoked from an interrupt or NMI handler, even if that interrupt or
> > > > + * NMI interrupted the CPU while it was deep within its idle loop.
> > > 
> > > But it is more than the idle loop, for ex. NOHZ_FULL CPUs with single task
> > > running could be idle from RCU's viewpoint? Could that be clarified more?
> > 
> > Perhaps something like this?
> > 
> >   * Although calls to rcu_is_watching() from most parts of the kernel
> >   * will return @true, there are important exceptions.  For example, if the
> >   * current CPU is deep within its idle loop, in kernel entry/exit code,
> >   * or offline, rcu_is_watching() will return @false.
> > 
> > (Where nohz_full CPUs are covered by kernel entry/exit code.)
> 
> To me, "kernel exit" does not immediately make the nohz_full CPU case
> obvious. But yes, your suggestion is an improvement so we can go with that.
> :)
> 
> Also because we agree on the changes, for next revision:
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Very good, thank you!  The update should appear shortly.  For some
definition of "shortly".  ;-)

							Thanx, Paul