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 <frede...@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
