Re: [PATCH v3 6/6] hisi_acc_vfio_pci: Add support for VFIO live migration

[Leon Romanovsky]

On Mon, Sep 27, 2021 at 12:01:19PM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 27, 2021 at 01:46:31PM +0000, Shameerali Kolothum Thodi wrote:
> 
> > > > > Nope, this is locked wrong and has no lifetime management.
> > > >
> > > > Ok. Holding the device_lock() sufficient here?
> > > 
> > > You can't hold a hisi_qm pointer with some kind of lifecycle
> > > management of that pointer. device_lock/etc is necessary to call
> > > pci_get_drvdata()
> > 
> > Since this migration driver only supports VF devices and the PF
> > driver will not be removed until all the VF devices gets removed,
> > is the locking necessary here?
> 
> Oh.. That is really busted up. pci_sriov_disable() is called under the
> device_lock(pf) and obtains the device_lock(vf).

Yes, indirectly, but yes.

> 
> This means a VF driver can never use the device_lock(pf), otherwise it
> can deadlock itself if PF removal triggers VF removal.

VF can use pci_dev_trylock() on PF to prevent PF removal.

> 
> But you can't access these members without using the device_lock(), as
> there really are no safety guarentees..
> 
> The mlx5 patches have this same sketchy problem.
> 
> We may need a new special function 'pci_get_sriov_pf_devdata()' that
> confirms the vf/pf relationship and explicitly interlocks with the
> pci_sriov_enable/disable instead of using device_lock()
> 
> Leon, what do you think?

I see pci_dev_lock() and similar functions, they are easier to
understand that specific pci_get_sriov_pf_devdata().

Thanks

> 
> Jason