linux-kernel - Re: [PATCH 2/2] rcu-tasks: Make synchronize_rcu_tasks

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Date:   Tue, 12 Jul 2022 10:37:09 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     frederic@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] rcu-tasks: Make synchronize_rcu_tasks_generic()
 no-ops on early booting

On Tue, Jul 12, 2022 at 04:26:06PM +0800, Zqiang wrote:
> When the rcu_scheduler_active variable is RCU_SCHEDULER_INACTIVE and not
> yet converted to RCU_SCHEDULER_INIT, there is only idle task, any legal
> call synchronize_rcu_tasks_generic() is a quiescent state. this commit
> make synchronize_rcu_tasks_generic() no-ops when the rcu_scheduler_active
> variable is RCU_SCHEDULER_INACTIVE.
> 
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> ---

It looks like this would be a good way to provide early boot access
to synchronize_rcu_tasks(), synchronize_rcu_tasks_rude(), and
synchronize_rcu_tasks_trace().

But do we really need early boot access to these functions?  As in has
the below WARN_ON() actually triggered?

And if it has triggered, and in a context that means that these functions
really are needed during early boot, how should the testing strategy
change to test these at the relevant portions of the boot sequence?

>From what I know, hitting these during early boot would indicate that
something was removing a trace during early boot, and I know of no way
to make that happen.  Hence my skepticism.  ;-)

But *if* this was really needed, this looks to be a reasonable way to
implement it.

							Thanx, Paul

>  kernel/rcu/tasks.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 469bf2a3b505..0237e765c28e 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -560,8 +560,9 @@ static int __noreturn rcu_tasks_kthread(void *arg)
>  static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
>  {
>  	/* Complain if the scheduler has not started.  */
> -	WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
> -			 "synchronize_rcu_tasks called too soon");
> +	if (WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
> +			 "synchronize_rcu_tasks called too soon"))
> +		return;
>  
>  	// If the grace-period kthread is running, use it.
>  	if (READ_ONCE(rtp->kthread_ptr)) {
> -- 
> 2.25.1
>