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SubjectRe: Race condition in vfio will cause deadwait
On Tue, 25 Jul 2017 21:30:04 +0800
Wanlong Gao <gaowanlong@huawei.com> wrote:

> From: Wen Congyang <wencongyang2@huawei.com>
>
> Hi, Alex Williamson
>
> When using vfio, we encounter a problem: too many lspci processes are blocked in D state.
> I analyzed all processes, and found one process is blocked in pci_dev_lock(), another
> process is blocked in vfio_del_group_put(). I checked the backtrace, and found the following
> race condition:
>
> Process A(use sysfs to unbind the device):
> device_release_driver()
> device_lock(dev)
> __device_release_driver()
> dev->bus->remove(dev) // pci_device_remove()
> drv->remove(pci_dev) // vfio_pci_remove()
> vfio_del_group_dev()
> vfio_device_put()
> wait vfio_device is remove from the group. Process B gets vfio_device,
> so we will wait it.
>
> Process B:
> vfio_group_fops_unl_ioctl()
> vfio_group_get_device_fd()
> vfio_device_get_from_name() // we get vfio_device here
> device->ops->open() // vfio_pci_open()
> vfio_pci_enable()
> pci_reset_function()
> pci_dev_reset(dev, 0)
> pci_dev_lock()
> pci_cfg_access_lock(dev) // After this, lspci will be blocked
> device_lock(&dev->dev) // the lock is hold by another process A
>
> Now, Process A is waiting Process B, and Process B is waiting Process A.
>
> I think we use pci_try_reset_function() to instead of pci_reset_function() can break this deadwait.

Thanks for your report and analysis, I think you're right. Using the
trylock version of reset seems like the best approach, I've also used
lock contention as a reason to fail the open path for the user here. I
would welcome your testing and feedback on the proposed patch:

https://lkml.org/lkml/2017/7/26/838

Thanks,
Alex

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