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

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Message-ID: <20220714033718.GT1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 13 Jul 2022 20:37:18 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Zhang, Qiang1" <qiang1.zhang@...el.com>
Cc:     "frederic@...nel.org" <frederic@...nel.org>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...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 Thu, Jul 14, 2022 at 01:53:20AM +0000, Zhang, Qiang1 wrote:
> 
> 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?
> 
> when the rcu_scheduler_active variable is RCU_SCHEDULER_INACTIVE,
> invoke synchronize_rcu_tasks_generic(), in addition to triggering a warning, 
> also need to make it return directly, if not, the rcu_tasks_one_gp() will be
> called directly, but due to the rtp structure's -> pregp_func is not initialized,
> A null pointer bug will appear.
> 
> But like said, I don't see the need to call synchronize_rcu_tasks_generic() on 
> early booting.  maybe this change is  not necessary.

Not yet, anyway.  And adding this would require more testing.

However, if the current warning does trigger, and the caller has a
legitimate reason for invoking this function so early, please remember
this patch.  ;-)

							Thanx, Paul

> Thanks
> Zqiang
> 
> >
> >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
> >