linux-kernel - Re: [PATCH tip/core/rcu 05/13] rcu: Always set .need_qs from __rcu_read

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Date:   Fri, 4 Sep 2020 06:41:42 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 05/13] rcu: Always set .need_qs from
 __rcu_read_lock() for strict GPs

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@...nel.org wrote:
> > From: "Paul E. McKenney" <paulmck@...nel.org>
> > 
> > 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@...gle.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> >  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
> >