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

Lai Jiangshan
2019-10-31T18:19:18Z

On 2019/10/31 9:43 下午, 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@linux.alibaba.com>
>> ---
>> 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
>>