On Fri, Sep 04, 2020 at 12:05:34PM +0800, Boqun Feng wrote:
> 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() ;-)
Because unless I am missing something subtle, if the .need_qs
flag is not set, execution is not guaranteed to reach
rcu_preempt_deferred_qs_irqrestore().
Thanx, Paul
> 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
> >
