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

[Paul E. McKenney]

On Fri, Nov 01, 2019 at 02:19:13AM +0800, Lai Jiangshan wrote:
> 
> 
> 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.

Thank you for checking, and yes, this code is a bit subtle.  So good
on you for digging into it!

							Thanx, Paul

> 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
> > >