Re: [PATCH 06/10] rcu: Make nocb_cb kthread parkable

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, May 13, 2020 at 06:47:10PM +0200, Frederic Weisbecker wrote:
> This will be necessary to correctly implement rdp de-offloading. We
> don't want rcu_do_batch() in nocb_cb kthread to race with local
> rcu_do_batch().
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Joel Fernandes <joel@joelfernandes.org>

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

 - Joel


> ---
>  kernel/rcu/tree_plugin.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 1dd3fdd675a1..43ecc047af26 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2104,7 +2104,9 @@ static void nocb_cb_wait(struct rcu_data *rdp)
>  	if (needwake_gp)
>  		rcu_gp_kthread_wake();
>  	swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
> -				 !READ_ONCE(rdp->nocb_cb_sleep));
> +				    !READ_ONCE(rdp->nocb_cb_sleep) ||
> +				    kthread_should_park());
> +
>  	if (!smp_load_acquire(&rdp->nocb_cb_sleep)) { /* VVV */
>  		/* ^^^ Ensure CB invocation follows _sleep test. */
>  		return;
> @@ -2125,6 +2127,8 @@ static int rcu_nocb_cb_kthread(void *arg)
>  	// if there are no more ready callbacks, waits for them.
>  	for (;;) {
>  		nocb_cb_wait(rdp);
> +		if (kthread_should_park())
> +			kthread_parkme();
>  		cond_resched_tasks_rcu_qs();
>  	}
>  	return 0;
> -- 
> 2.25.0
>