Re: [PATCH] VFIO: PCI: eliminate unnecessary overhead in vfio_pci_reflck_find

[linmiaohe]

On Wed, 6 Nov 2019 03:25
Alex Williamson wrote
>> From: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> 
>> The driver of the pci device may not equal to vfio_pci_driver, but we 
>> fetch vfio_device from pci_dev unconditionally in func 
>> vfio_pci_reflck_find. This overhead, such as the competition of 
>> vfio.group_lock, can be eliminated by check pci_dev_driver with 
>> vfio_pci_driver first.
>> 
>
>I believe this introduces a race.  When we hold a reference to the vfio device, an unbind from a vfio bus driver will be blocked in vfio_del_group_dev().  Therefore if we test the driver is vfio-pci while holding this reference, we know that it cannot be released and the device_data is a valid vfio_pci_device. Testing the driver prior to acquiring a vfio device reference is meaningless as we have no guarantee that the driver has not changed by the time we acquire a reference.  Are you actually seeing contention here or was this a code inspection optimization?  Thanks,
>
>Alex

Thanks for your reply and patient explanation.  It was a code inspection optimization, and obviously, I missed something. I'm really sorry about that.

Best wishes.
Have a nice day.