Re: [RFC PATCH 1/2] rcu: Add rcu_read_lock_notrace()

[Boqun Feng <boqun.feng@gmail.com>]

On Fri, Jun 13, 2025 at 05:22:17PM +0200, Sebastian Andrzej Siewior wrote:
> rcu_read_lock_notrace() follows preempt_disable_notrace(). The idea is
> provide a rcu_read_lock() version so that the preempt_disable_notrace()
> user (which use RCU-sched under the hood) can migrate to preemptible
> RCU.
> The first user should be tracing (tracepoint) which is using the
> _notrace variant.
> 
> rcu_read_lock_notrace() is a slim version of rcu_read_lock(). It simply
> increments/ decrements the counter. It does not emit any warnings if
> RCU_NEST_PMAX is exceeded or patricipates in STRICT_GRACE_PERIOD. It
> also does not participate in rcu_read_unlock_special() as it would if
> invoke from NMI.
> 

If it doesn't participate in rcu_read_unlock_special() then it may not
report a QS in time, right? For example:

	CPU 2			CPU 3
	=====
	rcu_read_lock_notrace();
				synchronize_rcu();
				// need all CPUs report a QS.
	rcu_read_unlock_notrace();
	// no rcu_read_unlock_special() so QS will only be reported next
	// time we if another RCU read-side critical section exits.

Wouldn't this be a problem?

Regards,
Boqun

> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
[...]
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 0b0f56f6abc85..02cccca917a22 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -420,6 +420,13 @@ void __rcu_read_lock(void)
>  }
>  EXPORT_SYMBOL_GPL(__rcu_read_lock);
>  
> +notrace void __rcu_read_lock_notrace(void)
> +{
> +	rcu_preempt_read_enter();
> +	barrier();  /* critical section after entry code. */
> +}
> +EXPORT_SYMBOL_GPL(__rcu_read_lock_notrace);
> +
>  /*
>   * Preemptible RCU implementation for rcu_read_unlock().
>   * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
> @@ -445,6 +452,13 @@ void __rcu_read_unlock(void)
>  }
>  EXPORT_SYMBOL_GPL(__rcu_read_unlock);
>  
> +notrace void __rcu_read_unlock_notrace(void)
> +{
> +	barrier();  // critical section before exit code.
> +	rcu_preempt_read_exit();
> +}
> +EXPORT_SYMBOL_GPL(__rcu_read_unlock_notrace);
> +
>  /*
>   * Advance a ->blkd_tasks-list pointer to the next entry, instead
>   * returning NULL if at the end of the list.
> -- 
> 2.49.0
> 
>