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Subject: Race condition in vfio will cause deadwait
CC: <gaowanlong@huawei.com>, <kvm@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>,
        "wencongyang (A)" <wencongyang2@huawei.com>,
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References: <03bf80d2-546d-614a-7af9-1f786fe01285@huawei.com>
To: Alex Williamson <alex.williamson@redhat.com>
From: Wanlong Gao <gaowanlong@huawei.com>
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Date: Tue, 25 Jul 2017 21:30:04 +0800
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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,
Wen Congyang