Re: [PATCH 02/11] rcu: fix bug when rcu_exp_handler() in nested interrupt

Lai Jiangshan
2019-11-01T02:29:47Z

On 2019/11/1 8:19 上午, Boqun Feng wrote:
> On Thu, Oct 31, 2019 at 11:52:58AM -0700, Paul E. McKenney wrote:
>> On Thu, Oct 31, 2019 at 11:14:23PM +0800, Lai Jiangshan wrote:
>>>
>>>
>>> On 2019/10/31 10:31 下午, Paul E. McKenney wrote:
>>>> On Thu, Oct 31, 2019 at 06:47:31AM -0700, Paul E. McKenney wrote:
>>>>> On Thu, Oct 31, 2019 at 10:07:57AM +0000, Lai Jiangshan wrote:
>>>>>> These is a possible bug (although which I can't triger yet)
>>>>>> since 2015 8203d6d0ee78
>>>>>> (rcu: Use single-stage IPI algorithm for RCU expedited grace period)
>>>>>>
>>>>>> rcu_read_unlock()
>>>>>> ->rcu_read_lock_nesting = -RCU_NEST_BIAS;
>>>>>> interrupt(); // before or after rcu_read_unlock_special()
>>>>>> rcu_read_lock()
>>>>>> fetch some rcu protected pointers
>>>>>> // exp GP starts in other cpu.
>>>>>> some works
>>>>>> NESTED interrupt for rcu_exp_handler();
>>>>
>>>> Also, which platforms support nested interrupts? Last I knew, this was
>>>> prohibited.
>>>>
>>>>>> report exp qs! BUG!
>>>>>
>>>>> Why would a quiescent state for the expedited grace period be reported
>>>>> here? This CPU is still in an RCU read-side critical section, isn't it?
>>>>
>>>> And I now see what you were getting at here. Yes, the current code
>>>> assumes that interrupt-disabled regions, like hardware interrupt
>>>> handlers, cannot be interrupted. But if interrupt-disabled regions such
>>>> as hardware interrupt handlers can be interrupted (as opposed to being
>>>> NMIed), wouldn't that break a whole lot of stuff all over the place in
>>>> the kernel? So that sounds like an arch bug to me.
>>>
>>> I don't know when I started always assuming hardware interrupt
>>> handler can be nested by (other) interrupt. I can't find any
>>> documents say Linux don't allow nested interrupt handler.
>>> Google search suggests the opposite.
>
> FWIW, there is a LWN article talking about we disallow interrupt nesting
> in *most* cases:
>
> https://lwn.net/Articles/380931/

Much thanks for the information!

>
> , that's unless a interrupt handler explicitly calls
> local_irq_enable_in_hardirq(), it remains irq disabled, which means no
> nesting interrupt allowed.
>
Even so the problem here will be fixed by patch7/8.

>
>>
>> The results I am seeing look to be talking about threaded interrupt
>> handlers, which indeed can be interrupted by hardware interrupts. As can
>> softirq handlers. But these are not examples of a hardware interrupt
>> handler being interrupted by another hardware interrupt. For that to
>> work reasonably, something like a system priority level is required,
>> as in the old DYNIX/ptx kernel, or, going even farther back, DEC's RT-11.
>>
>>> grep -rIni nested Documentation/memory-barriers.txt Documentation/x86/
>>> It still have some words about nested interrupt handler.
>>
>> Some hardware does not differentiate between interrupts and exceptions,
>> for example, an illegal-instruction trap within an interrupt handler
>> might look in some ways like a nested interrupt.
>>
>>> The whole patchset doesn't depend on this patch, and actually
>>> it is reverted later in the patchset. Dropping this patch
>>> can be an option for next round.
>>
>> Sounds like a plan!
>>
>> Thanx, Paul
>>
> [...]
>