Re: [PATCH tip/core/rcu 05/13] rcu: Always set .need_qs from __rcu_read_lock() for strict GPs

[Boqun Feng]

Hi Paul,

On Mon, Aug 31, 2020 at 11:11:12AM -0700, paulmck@xxxxxxxxxx wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> 
> The ->rcu_read_unlock_special.b.need_qs field in the task_struct
> structure indicates that the RCU core needs a quiscent state from the
> corresponding task.  The __rcu_read_unlock() function checks this (via
> an eventual call to rcu_preempt_deferred_qs_irqrestore()), and if set
> reports a quiscent state immediately upon exit from the outermost RCU
> read-side critical section.
> 
> Currently, this flag is only set when the scheduling-clock interrupt
> decides that the current RCU grace period is too old, as in about
> one full second too old.  But if the kernel has been built with
> CONFIG_RCU_STRICT_GRACE_PERIOD=y, we clearly do not want to wait that
> long.  This commit therefore sets the .need_qs field immediately at the
> start of the RCU read-side critical section from within __rcu_read_lock()
> in order to unconditionally enlist help from __rcu_read_unlock().
> 

So why not make rcu_preempt_deferred_qs_irqrestore() always treat
need_qs is true if CONFIG_RCU_STRICT_GRACE_PERIOD = y? IOW:

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 982fc5be5269..2a9f31545453 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -449,6 +449,8 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
 	 * t->rcu_read_unlock_special cannot change.
 	 */
 	special = t->rcu_read_unlock_special;
+	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
+		special.b.need_qs = true;
 	rdp = this_cpu_ptr(&rcu_data);
 	if (!special.s && !rdp->exp_deferred_qs) {
 		local_irq_restore(flags);

, and in this way, you can save one store for each rcu_read_lock() ;-)

Regards,
Boqun

> But note the additional check for rcu_state.gp_kthread, which prevents
> attempts to awaken RCU's grace-period kthread during early boot before
> there is a scheduler.  Leaving off this check results in early boot hangs.
> So early that there is no console output.  Thus, this additional check
> fails until such time as RCU's grace-period kthread has been created,
> avoiding these empty-console hangs.
> 
> Reported-by Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
>  kernel/rcu/tree_plugin.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 44cf77d..668bbd2 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -376,6 +376,8 @@ void __rcu_read_lock(void)
>  	rcu_preempt_read_enter();
>  	if (IS_ENABLED(CONFIG_PROVE_LOCKING))
>  		WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
> +	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
> +		WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
>  	barrier();  /* critical section after entry code. */
>  }
>  EXPORT_SYMBOL_GPL(__rcu_read_lock);
> -- 
> 2.9.5
>