Re: [PATCH] irqchip/gic-v3-its: Lock its device list during find and create its device
From: Zheng Xiang
Date: Mon Jan 28 2019 - 02:14:48 EST
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.
>
> The whole RID aliasing is such a mess, I wish we never supported
> it. Anyway, comments below.
>
>>
>> Signed-off-by: Zheng Xiang <zhengxiang9@xxxxxxxxxx>
>> ---
>> 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