Re: [PATCH rcu 2/6] rcu: Clarify rcu_is_watching() kernel-doc comment

[Joel Fernandes]

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@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
>   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.

>    *
> - * 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?

thanks,

 - Joel


>    *
>    * Make notrace because it can be called by the internal functions of
>    * ftrace, and making this notrace removes unnecessary recursion calls.