Date:   Mon, 28 Jan 2019 13:51:36 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Zheng Xiang <zhengxiang9@...wei.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        jason@...edaemon.net, wanghaibin.wang@...wei.com
Subject: Re: [PATCH] irqchip/gic-v3-its: Lock its device list during find and
 create its device

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@...wei.com> 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).

> 
>>
>> The whole RID aliasing is such a mess, I wish we never supported
>> it. Anyway, comments below.
>>
>>>
>>> Signed-off-by: Zheng Xiang <zhengxiang9@...wei.com>
>>> ---
>>>  drivers/irqchip/irq-gic-v3-its.c | 52 +++++++++++++++-------------------------
>>>  1 file changed, 19 insertions(+), 33 deletions(-)
>>>
>>> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
>>> index db20e99..397edc8 100644
>>> --- a/drivers/irqchip/irq-gic-v3-its.c
>>> +++ b/drivers/irqchip/irq-gic-v3-its.c
>>> @@ -2205,25 +2205,6 @@ static void its_cpu_init_collections(void)
>>>  	raw_spin_unlock(&its_lock);
>>>  }
>>>  
>>> -static struct its_device *its_find_device(struct its_node *its, u32 dev_id)
>>> -{
>>> -	struct its_device *its_dev = NULL, *tmp;
>>> -	unsigned long flags;
>>> -
>>> -	raw_spin_lock_irqsave(&its->lock, flags);
>>> -
>>> -	list_for_each_entry(tmp, &its->its_device_list, entry) {
>>> -		if (tmp->device_id == dev_id) {
>>> -			its_dev = tmp;
>>> -			break;
>>> -		}
>>> -	}
>>> -
>>> -	raw_spin_unlock_irqrestore(&its->lock, flags);
>>> -
>>> -	return its_dev;
>>> -}
>>> -
>>>  static struct its_baser *its_get_baser(struct its_node *its, u32 type)
>>>  {
>>>  	int i;
>>> @@ -2321,7 +2302,7 @@ static bool its_alloc_vpe_table(u32 vpe_id)
>>>  static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
>>>  					    int nvecs, bool alloc_lpis)
>>>  {
>>> -	struct its_device *dev;
>>> +	struct its_device *dev = NULL, *tmp;
>>>  	unsigned long *lpi_map = NULL;
>>>  	unsigned long flags;
>>>  	u16 *col_map = NULL;
>>> @@ -2331,6 +2312,24 @@ static struct its_device *its_create_device(struct its_node *its, u32 dev_id,
>>>  	int nr_ites;
>>>  	int sz;
>>>  
>>> +	raw_spin_lock_irqsave(&its->lock, flags);
>>> +	list_for_each_entry(tmp, &its->its_device_list, entry) {
>>> +		if (tmp->device_id == dev_id) {
>>> +			dev = tmp;
>>> +			break;
>>> +		}
>>> +	}
>>> +	if (dev) {
>>> +		/*
>>> +		 * We already have seen this ID, probably through
>>> +		 * another alias (PCI bridge of some sort). No need to
>>> +		 * create the device.
>>> +		 */
>>> +		pr_debug("Reusing ITT for devID %x\n", dev_id);
>>> +		raw_spin_unlock_irqrestore(&its->lock, flags);
>>> +		return dev;
>>> +	}
>>> +
>>>  	if (!its_alloc_device_table(its, dev_id))
>>
>> You're now performing all sort of allocations in an atomic context,
>> which is pretty horrible (and the kernel will shout at you for doing
>> so).
>>
>> We could probably keep the current logic and wrap it around a mutex
>> instead, which would give us the appropriate guarantees WRT allocations.
>> Something along those lines (untested):>
>> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
>> index db20e992a40f..99feb62e63ba 100644
>> --- a/drivers/irqchip/irq-gic-v3-its.c
>> +++ b/drivers/irqchip/irq-gic-v3-its.c
>> @@ -97,9 +97,14 @@ struct its_device;
>>   * The ITS structure - contains most of the infrastructure, with the
>>   * top-level MSI domain, the command queue, the collections, and the
>>   * list of devices writing to it.
>> + *
>> + * alloc_lock has to be taken for any allocation that can happen at
>> + * run time, while the spinlock must be taken to parse data structures
>> + * such as the device list.
>>   */
>>  struct its_node {
>>  	raw_spinlock_t		lock;
>> +	struct mutex		alloc_lock;
>>  	struct list_head	entry;
>>  	void __iomem		*base;
>>  	phys_addr_t		phys_base;
>> @@ -2421,6 +2426,7 @@ static int its_msi_prepare(struct irq_domain *domain, struct device *dev,
>>  	struct its_device *its_dev;
>>  	struct msi_domain_info *msi_info;
>>  	u32 dev_id;
>> +	int err = 0;
>>  
>>  	/*
>>  	 * We ignore "dev" entierely, and rely on the dev_id that has
>> @@ -2443,6 +2449,7 @@ static int its_msi_prepare(struct irq_domain *domain, struct device *dev,
>>  		return -EINVAL;
>>  	}
>>  
>> +	mutex_lock(&its->alloc_lock);
>>  	its_dev = its_find_device(its, dev_id);
>>  	if (its_dev) {
>>  		/*
>> @@ -2455,11 +2462,14 @@ static int its_msi_prepare(struct irq_domain *domain, struct device *dev,
>>  	}
>>  
>>  	its_dev = its_create_device(its, dev_id, nvec, true);
>> -	if (!its_dev)
>> -		return -ENOMEM;
>> +	if (!its_dev) {
>> +		err = -ENOMEM;
>> +		goto out;
>> +	}
>>  
>>  	pr_debug("ITT %d entries, %d bits\n", nvec, ilog2(nvec));
>>  out:
>> +	mutex_unlock(&its->alloc_lock);
>>  	info->scratchpad[0].ptr = its_dev;
>>  	return 0;
> 
> Should it return *err* here?

Absolutely. Does it fix the problem for you?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

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