Re: [PATCH 04/11] rcu: cleanup rcu_preempt_deferred_qs()

[Lai Jiangshan]

On 2019/10/31 11:07 äå, Paul E. McKenney wrote:
> On Thu, Oct 31, 2019 at 10:35:22PM +0800, Lai Jiangshan wrote:
> > On 2019/10/31 10:10 äå, 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.

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.

Thanks
Lai

> 							Thanx, Paul
>
> > > > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > >    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