linux-kernel - Re: [PATCH 04/11] rcu: cleanup rcu_preempt_deferred

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Date:   Thu, 31 Oct 2019 08:07:19 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 04/11] rcu: cleanup rcu_preempt_deferred_qs()

On Thu, Oct 31, 2019 at 10:35:22PM +0800, Lai Jiangshan wrote:
> On 2019/10/31 10:10 下午, Paul E. McKenney wrote:
> > On Thu, Oct 31, 2019 at 10:07:59AM +0000, Lai Jiangshan wrote:
> > > Don't need to set ->rcu_read_lock_nesting negative, irq-protected
> > > rcu_preempt_deferred_qs_irqrestore() doesn't expect
> > > ->rcu_read_lock_nesting to be negative to work, it even
> > > doesn't access to ->rcu_read_lock_nesting any more.
> > > 
> > > It is true that NMI over rcu_preempt_deferred_qs_irqrestore()
> > > may access to ->rcu_read_lock_nesting, but it is still safe
> > > since rcu_read_unlock_special() can protect itself from NMI.
> > 
> > Hmmm...  Testing identified the need for this one.  But I will wait for
> > your responses on the earlier patches before going any further through
> > this series.
> 
> Hmmm... I was wrong, it should be after patch7 to avoid
> the scheduler deadlock.

I was wondering about that.  ;-)

							Thanx, Paul

> > > Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> > > ---
> > >   kernel/rcu/tree_plugin.h | 5 -----
> > >   1 file changed, 5 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index 82595db04eec..9fe8138ed3c3 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -555,16 +555,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
> > >   static void rcu_preempt_deferred_qs(struct task_struct *t)
> > >   {
> > >   	unsigned long flags;
> > > -	bool couldrecurse = t->rcu_read_lock_nesting >= 0;
> > >   	if (!rcu_preempt_need_deferred_qs(t))
> > >   		return;
> > > -	if (couldrecurse)
> > > -		t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
> > >   	local_irq_save(flags);
> > >   	rcu_preempt_deferred_qs_irqrestore(t, flags);
> > > -	if (couldrecurse)
> > > -		t->rcu_read_lock_nesting += RCU_NEST_BIAS;
> > >   }
> > >   /*
> > > -- 
> > > 2.20.1
> > >