Re: [PATCH] irqchip/gic-v3-its: Lock its device list during find and create its device

[Zheng Xiang]

On 2019/2/1 17:28, Marc Zyngier wrote:
> On 01/02/2019 06:41, Zheng Xiang wrote:
>>
>> On 2019/1/31 23:12, Marc Zyngier wrote:
>>> Hi Zeng,
>>>
>>> On 31/01/2019 14:47, Zheng Xiang wrote:
>>>> Hi Marc,
>>>>
>>>> On 2019/1/29 13:42, Zheng Xiang wrote:
>>>>> On 2019/1/28 21:51, Marc Zyngier wrote:
>>>>>> On 28/01/2019 07:13, Zheng Xiang wrote:
>>>>>>> Hi Marc,
>>>>>>>
>>>>>>> Thanks for your review.
>>>>>>>
>>>>>>> On 2019/1/26 19:38, Marc Zyngier wrote:
>>>>>>>> Hi Zheng,
>>>>>>>>
>>>>>>>> On Sat, 26 Jan 2019 06:16:24 +0000,
>>>>>>>> Zheng Xiang <zhengxiang9@xxxxxxxxxx> wrote:
>>>>>>>>>
>>>>>>>>> Currently each PCI device under a PCI Bridge shares the same device id
>>>>>>>>> and ITS device. Assume there are two PCI devices call its_msi_prepare
>>>>>>>>> concurrently and they are both going to find and create their ITS
>>>>>>>>> device. There is a chance that the later one couldn't find ITS device
>>>>>>>>> before the other one creating the ITS device. It will cause the later
>>>>>>>>> one to create a different ITS device even if they have the same
>>>>>>>>> device_id.
>>>>>>>>
>>>>>>>> Interesting finding. Is this something you've actually seen in practice
>>>>>>>> with two devices being probed in parallel? Or something that you found
>>>>>>>> by inspection?
>>>>>>>
>>>>>>> Yes, I find this problem after analyzing the reason of VM hung. At last, I
>>>>>>> find that the virtio-gpu cannot receive the MSI interrupts due to sharing
>>>>>>> a same event_id as virtio-serial.
>>>>>>>
>>>>>>> See https://lkml.org/lkml/2019/1/10/299 for the bug report.
>>>>>>>
>>>>>>> This problem can be reproducted with high probability by booting a Qemu/KVM
>>>>>>> VM with a virtio-serial controller and a virtio-gpu adding to a PCI Bridge
>>>>>>> and also adding some delay before creating ITS device.
>>>>>>
>>>>>> Fair enough. Do you mind sharing your QEMU command line? It'd be useful
>>>>>> if I could reproduce it here (and would give me a way to check that it
>>>>>> doesn't regress).
>>>>>
>>>>
>>>> Have you reproduced it with my QEMU command line?
>>>>
>>>> If so, should I send a V2 patch with your suggestion?
>>>
>>> I've queued the following, much more complete patch:
>>>
>>> https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/commit/?h=irq/irqchip-next&id=9791ec7df0e7b4d80706ccea8f24b6542f6059e9
>>>
>>> Can you check that it works for you? I didn't manage to get the right
>>> timing conditions, but I also had issues getting virtio-gpu running on
>>> my TX2, so one might explain the other.
>>>
>>
>> It works for my case, but I worried about the below lines which may
>> cause memory leak.
>>
>> @@ -2627,8 +2640,14 @@ static void its_irq_domain_free(struct irq_domain *domain, unsigned int virq,
>>  		irq_domain_reset_irq_data(data);
>>  	}
>>
>> -	/* If all interrupts have been freed, start mopping the floor */
>> -	if (bitmap_empty(its_dev->event_map.lpi_map,
>> +	mutex_lock(&its->dev_alloc_lock);
>> +
>> +	/*
>> +	 * If all interrupts have been freed, start mopping the
>> +	 * floor. This is conditionned on the device not being shared.
>> +	 */
>> +	if (!its_dev->shared &&
>> +	    bitmap_empty(its_dev->event_map.lpi_map,
>>  			 its_dev->event_map.nr_lpis)) {
>>  		its_lpi_free(its_dev->event_map.lpi_map,
>>  			     its_dev->event_map.lpi_base,
>>
>> It seems that the shared its_dev would never be freed since the value of
>> its_dev->shared is always *true*.
> 
> Yes, and that is on purpose. As we don't refcount the number of
> interrupts that have been requested in the prepare phase, there is a
> race between free and alloc. We can have the following situation:
> 
> CPU0:               CPU1:
> 
> msi_prepare:
> mutex_lock()
> find device()
>   -> found
> store its_dev
> mutex_unlock()
> 
>                     its_irq_domain_free:
>                     mutex_lock()
>                     free_device()
>                     mutex_unlock()
> 
> its_irq_domain_alloc:
> use its_dev -> boom.
> 
> 
> So the trick is not to free the its_dev structure if it shares a devid.
> It is not really a leak, as the next device sharing the same devid will
> pick up the same structure.
> 
> Does it make sense?

Yes, Thanks a lot!


-- 

Thanks,
Xiang