On Tue, 25 Jul 2017 21:30:04 +0800 Wanlong Gao <gaowanlong@xxxxxxxxxx> wrote: > From: Wen Congyang <wencongyang2@xxxxxxxxxx> > > 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
