Re: [PATCH 2/2] rcu: Robustify rcu_is_cpu_rrupt_from_idle()

[Joel Fernandes <joelagnelf@nvidia.com>]

On 3/18/2025 2:56 PM, Frederic Weisbecker wrote:
> RCU relies on the context tracking nesting counter in order to determine
> if it is running in extended quiescent state.
> 
> However the context tracking nesting counter is not completely
> synchronized with the actual context tracking state:
> 
> * The nesting counter is set to 1 or incremented further _after_ the
>   actual state is set to RCU not watching.

I agree with patch, but this line is a bit confusing ->nesting is set to 1
*after* the RCU state is set to "watching".  Did you mean "watching" ?

But I think you meant "After RCU transitions from a state of not-watching to
watching' instead of 'actual state is set to RCU not watching'..

ct_kernel_entry():

	// RCU is not watching here ...
	ct_kernel_enter_state(offset);
	// ... but is watching here.
	WRITE_ONCE(ct->nesting, 1);

>    (then we know for sure we interrupted RCU not watching)
> 
> * The nesting counter is set to 0 or decremented further _before_ the
>   actual state is set to RCU watching.
> 
> Therefore it is safe to assume that if ct_nesting() > 0, RCU is not
> watching. But if ct_nesting() <= 0, RCU is watching except for a tiny
> window.
> 
> This hasn't been a problem so far because rcu_is_cpu_rrupt_from_idle()
> has only been called from interrupts. However the code is confusing

Agreed, and I could also see the existing code's snippet:
	WARN_ON_ONCE(!nesting && !is_idle_task(current));

.. not working if this function were to be called from non-interrupt kernel context.


> and abuses the role of the context tracking nesting counter while there
> are more accurate indicators available.
> 
> Clarify and robustify accordingly.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>  kernel/rcu/tree.c | 27 +++++++++++++++++----------
>  1 file changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 79dced5fb72e..90c43061c981 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -367,7 +367,7 @@ EXPORT_SYMBOL_GPL(rcu_momentary_eqs);
>   */
>  static int rcu_is_cpu_rrupt_from_idle(void)
>  {
> -	long nesting;
> +	long nmi_nesting = ct_nmi_nesting();
>  
>  	/*
>  	 * Usually called from the tick; but also used from smp_function_call()
> @@ -379,21 +379,28 @@ static int rcu_is_cpu_rrupt_from_idle(void)
>  	/* Check for counter underflows */
>  	RCU_LOCKDEP_WARN(ct_nesting() < 0,
>  			 "RCU nesting counter underflow!");
> -	RCU_LOCKDEP_WARN(ct_nmi_nesting() <= 0,
> -			 "RCU nmi_nesting counter underflow/zero!");
>  
> -	/* Are we at first interrupt nesting level? */
> -	nesting = ct_nmi_nesting();
> -	if (nesting > 1)
> +	/* Non-idle interrupt or nested idle interrupt */
> +	if (nmi_nesting > 1)
>  		return false;
>  
>  	/*
> -	 * If we're not in an interrupt, we must be in the idle task!
> +	 * Non nested idle interrupt (interrupting section where RCU
> +	 * wasn't watching).
>  	 */
> -	WARN_ON_ONCE(!nesting && !is_idle_task(current));
> +	if (nmi_nesting == 1)
> +		return true;
>  
> -	/* Does CPU appear to be idle from an RCU standpoint? */
> -	return ct_nesting() == 0;
> +	/* Not in an interrupt */
> +	if (!nmi_nesting) {
> +		RCU_LOCKDEP_WARN(!in_task() || !is_idle_task(current),
> +				 "RCU nmi_nesting counter not in idle task!");
> +		return !rcu_is_watching_curr_cpu();

Makes sense to me and it is also consistent with rcu_watching_snap_in_eqs().

thanks,

 - Joel