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

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Message-ID: <20191031224525.GC20975@paulmck-ThinkPad-P72>
Date:   Thu, 31 Oct 2019 15:45:25 -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 Fri, Nov 01, 2019 at 02:33:19AM +0800, Lai Jiangshan wrote:
> 
> 
> On 2019/10/31 11:07 下午, Paul E. McKenney wrote:
> > 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.  ;-)
> 
> This patch was split from the core patch(patch8: don't use negative
> ->rcu_read_lock_nesting).
> 
> When I reordered "fixing something" as patch1/2, I reordered
> it close to the patch of clean up rcu_preempt_deferred_qs_irqrestore
> caused this mistake.

OK.

> I will reorder it back later and "fixing something" is fixing
> nothing and I will drop patch 1/2. Could you continue to review
> further through this series please? Sorry for any mistakes.

I will take at least a quick look in the morning, which will be
about ten hours from now.

							Thanx, Paul

> Thanks
> Lai
> 
> > 							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
> > > > >