Re: [Intel-gfx] [PATCH v4 16/19] vfio: Add VFIO_DEVICE_BIND_IOMMUFD

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Feb 24, 2023 at 12:58:22PM +0800, Yan Zhao wrote:
> On Wed, Feb 22, 2023 at 08:59:51AM -0400, Jason Gunthorpe wrote:
> > On Wed, Feb 22, 2023 at 07:44:12AM +0000, Liu, Yi L wrote:
> > > > From: Tian, Kevin <kevin.tian@intel.com>
> > > > Sent: Wednesday, February 22, 2023 3:40 PM
> > > > 
> > > > > From: Liu, Yi L <yi.l.liu@intel.com>
> > > > > Sent: Tuesday, February 21, 2023 11:48 AM
> > > > >
> > > > > +
> > > > > +void vfio_device_cdev_close(struct vfio_device_file *df)
> > > > > +{
> > > > > +	struct vfio_device *device = df->device;
> > > > > +
> > > > > +	mutex_lock(&device->dev_set->lock);
> > > > > +	if (!smp_load_acquire(&df->access_granted)) {
> > > > 
> > > > there is no contention with another one changing this flag at this
> > > > point so directly accessing it is fine.
> > > 
> > > make sense. 
> > 
> > Have to use READ_ONCE though
> >
> Just a curious question:
> given df->access_granted is now written with device->dev_set->lock held and
> also read with this lock held in vfio_device_cdev_close(), is READ_ONCE
> still required? And what about df->iommufd ?

No, if the writer is under a lock held by the reader then it is always
OK to use naked read. Best to document it with a comment

Jason