Re: [PATCH 01/11] rcu: avoid leaking exp_deferred_qs into next GP

[Lai Jiangshan]

On 2019/10/31 9:43 äå, Paul E. McKenney wrote:
> On Thu, Oct 31, 2019 at 10:07:56AM +0000, Lai Jiangshan wrote:
> > If exp_deferred_qs is incorrectly set and leaked to the next
> > exp GP, it may cause the next GP to be incorrectly prematurely
> > completed.
>
> Could you please provide the sequence of events leading to a such a
> failure?

I just felt nervous with "leaking" exp_deferred_qs.
I didn't careful consider the sequence of events.

Now it proves that I must have misunderstood the exp_deferred_qs.
So call "leaking" is wrong concept, preempt_disable()
is considered as rcu_read_lock() and exp_deferred_qs
needs to be set.

Thanks
Lai

============don't need to read:

read_read_lock()
// other cpu start exp GP_A
preempt_schedule() // queue itself
read_read_unlock() //report qs, other cpu is sending ipi to me
preempt_disable
  rcu_exp_handler() interrupt for GP_A and leave a exp_deferred_qs
  // exp GP_A finished
  ---------------above is one possible way to leave a exp_deferred_qs
preempt_enable()
 interrupt before preempt_schedule()
  read_read_lock()
  read_read_unlock()
   NESTED interrupt when nagative rcu_read_lock_nesting
    read_read_lock()
    // other cpu start exp GP_B
    NESTED interrupt for rcu_flavor_sched_clock_irq()
     report exq qs since rcu_read_lock_nesting <0 and \
     exp_deferred_qs is true
    // exp GP_B complete
    read_read_unlock()

This plausible sequence relies on NESTED interrupt too,
and can be avoided by patch2 if NESTED interrupt were allowed.

> Also, did you provoke such a failure in testing?  If so, an upgrade
> to rcutorture would be good, so please tell me what you did to make
> the failure happen.
>
> I do like the reduction in state space, but I am a bit concerned about
> the potential increase in contention on rnp->lock.  Thoughts?
>
> 							Thanx, Paul
>
> > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > ---
> >   kernel/rcu/tree_exp.h | 23 ++++++++++++++---------
> >   1 file changed, 14 insertions(+), 9 deletions(-)
> >
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index a0e1e51c51c2..6dec21909b30 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -603,6 +603,18 @@ static void rcu_exp_handler(void *unused)
> >   	struct rcu_node *rnp = rdp->mynode;
> >   	struct task_struct *t = current;
> >   
> > +	/*
> > +	 * Note that there is a large group of race conditions that
> > +	 * can have caused this quiescent state to already have been
> > +	 * reported, so we really do need to check ->expmask first.
> > +	 */
> > +	raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > +	if (!(rnp->expmask & rdp->grpmask)) {
> > +		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> > +		return;
> > +	}
> > +	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> > +
> >   	/*
> >   	 * First, the common case of not being in an RCU read-side
> >   	 * critical section.  If also enabled or idle, immediately
> > @@ -628,17 +640,10 @@ static void rcu_exp_handler(void *unused)
> >   	 * a future context switch.  Either way, if the expedited
> >   	 * grace period is still waiting on this CPU, set ->deferred_qs
> >   	 * so that the eventual quiescent state will be reported.
> > -	 * Note that there is a large group of race conditions that
> > -	 * can have caused this quiescent state to already have been
> > -	 * reported, so we really do need to check ->expmask.
> >   	 */
> >   	if (t->rcu_read_lock_nesting > 0) {
> > -		raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > -		if (rnp->expmask & rdp->grpmask) {
> > -			rdp->exp_deferred_qs = true;
> > -			t->rcu_read_unlock_special.b.exp_hint = true;
> > -		}
> > -		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> > +		rdp->exp_deferred_qs = true;
> > +		WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> >   		return;
> >   	}
> >   
> > --
> > 2.20.1