Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs()

[Lai Jiangshan]

On 2019/11/3 10:01 äå, Boqun Feng wrote:
> Hi Jiangshan,
>
>
> I haven't checked the correctness of this patch carefully, but..
>
>
> On Sat, Nov 02, 2019 at 12:45:54PM +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.
>
> rcu_preempt_deferred_qs_irqrestore() will report RCU qs, and may
> eventually call swake_up() or its friends to wake up, say, the gp
> kthread, and the wake up functions could go into the scheduler code
> path which might have RCU read-side critical section in it, IOW,
> accessing ->rcu_read_lock_nesting.

Sure, thank you for pointing it out.

I should rewrite the changelog in next round. Like this:

rcu: cleanup rcu_preempt_deferred_qs()

IRQ-protected rcu_preempt_deferred_qs_irqrestore() itself doesn't
expect ->rcu_read_lock_nesting to be negative to work.

There might be RCU read-side critical section in it (from wakeup()
or so), 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
will ensure that ->rcu_read_unlock_special is zero and these RCU
read-side critical sections will not call rcu_read_unlock_special().

Thanks
Lai

===
PS: Were 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once)
not applied earlier, it will be protected by previous patch (patch1)
in this series
"rcu: use preempt_count to test whether scheduler locks is held"
when rcu_read_unlock_special() is called.



> Again, haven't checked closely, but this argument in the commit log
> seems untrue.
>
> Regards,
> Boqun
>
> > 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.
> >
> > 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 aba5896d67e3..2fab8be2061f 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -552,16 +552,11 @@ 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