On 2022/6/21 10:49, Tian, Kevin wrote:
From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
Sent: Tuesday, June 21, 2022 9:32 AM
On 2022/6/21 04:13, Matthew Rosato wrote:
On 6/20/22 4:54 AM, Yi Liu wrote:
No need to protect open_count with group_rwsem
Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
cc: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Signed-off-by: Yi Liu <yi.l.liu@xxxxxxxxx>
Seems pretty harmless as-is, but you are correct group_rwsem can be
dropped
earlier; we do not protect the count with group_rwsem elsewhere (see
vfio_device_fops_release as a comparison, where we already drop
group_rwsem
before open_count--)
yes. this is exactly how I found it. Normally, I compare the err handling
path with the release function to see if they are aligned. :-)
In this case we don't need a FIX tag. It's kind of optimization.
ok.
FWIW, this change now also drops group_rswem before setting device-
kvm =
NULL, but that's also OK (again, just like vfio_device_fops_release) --
While the setting of device->kvm before open_device is technically done
while holding the group_rwsem, this is done to protect the group kvm
value
we are copying from, and we should not be relying on that to protect the
contents of device->kvm; instead we assume this value will not change until
after the device is closed and while under the dev_set->lock.
yes. set device->kvm to be NULL has no need to hold group_rwsem. BTW. I
also doubt whether the device->ops->open_device(device) and
device->ops->close_device(device) should be protected by group_rwsem or
not. seems not, right? group_rwsem protects the fields under vfio_group.
For the open_device/close_device() device->dev_set->lock is enough. Maybe
another nit fix.
group->rwsem is to protect device->group->kvm from being changed
by vfio_file_set_kvm() before it is copied to device->kvm.
yes. this is why vfio_device_open() holds the read lock of group_rwsem
around the device->group->kvm copy. However, for the open_device(),
callback, I don't think it is necessary to be protected by the group_rwsem
lock.
--
Regards,
Yi Liu
