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

Lai Jiangshan
2019-10-31T18:33:24Z

On 2019/10/31 11:07 下午, Paul E. McKenney wrote:
> On Thu, Oct 31, 2019 at 10:35:22PM +0800, Lai Jiangshan wrote:
>> On 2019/10/31 10:10 下午, 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@linux.alibaba.com>
>>>> ---
>>>> 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
>>>>