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Subject: Re: [PATCH] irqchip/gic-v3-its: Lock its device list during find and
 create its device
To:     Marc Zyngier <marc.zyngier@arm.com>
CC:     <linux-kernel@vger.kernel.org>, <tglx@linutronix.de>,
        <jason@lakedaemon.net>, <wanghaibin.wang@huawei.com>
References: <20190126061624.5260-1-zhengxiang9@huawei.com>
 <86bm438x8n.wl-marc.zyngier@arm.com>
From:   Zheng Xiang <zhengxiang9@huawei.com>
Message-ID: <27e0b952-111f-f221-bcd7-1a7ceb2840b5@huawei.com>
Date:   Mon, 28 Jan 2019 15:13:01 +0800
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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@huawei.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.

> 
> The whole RID aliasing is such a mess, I wish we never supported
> it. Anyway, comments below.
> 
>>
>> Signed-off-by: Zheng Xiang <zhengxiang9@huawei.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?

>  }
> @@ -3516,6 +3526,7 @@ static int __init its_probe_one(struct resource *res,
>  	}
>  
>  	raw_spin_lock_init(&its->lock);
> +	mutex_init(&its->alloc_lock);
>  	INIT_LIST_HEAD(&its->entry);
>  	INIT_LIST_HEAD(&its->its_device_list);
>  	typer = gic_read_typer(its_base + GITS_TYPER);
> 
> I still feel that the issue you're seeing here is much more generic.
> Overall, there is no guarantee that for a given MSI domain, no two
> allocation will take place in parallel, and maybe that's what we should
> enforce instead.
> 
> Thanks,
> 
> 	M.
> 
-- 

Thanks,
Xiang