[PATCH 5.10 389/545] vfio: Remove extra put/gets around vfio_device->group

Greg Kroah-Hartman
2022-08-19T16:34:50Z

From: Jason Gunthorpe <jgg@nvidia.com>

[ Upstream commit e572bfb2b6a83b05acd30c03010e661b1967960f ]

The vfio_device->group value has a get obtained during
vfio_add_group_dev() which gets moved from the stack to vfio_device->group
in vfio_group_create_device().

The reference remains until we reach the end of vfio_del_group_dev() when
it is put back.

Thus anything that already has a kref on the vfio_device is guaranteed a
valid group pointer. Remove all the extra reference traffic.

It is tricky to see, but the get at the start of vfio_del_group_dev() is
actually pairing with the put hidden inside vfio_device_put() a few lines
below.

A later patch merges vfio_group_create_device() into vfio_add_group_dev()
which makes the ownership and error flow on the create side easier to
follow.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Message-Id: <1-v3-225de1400dfc+4e074-vfio1_jgg@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/vfio/vfio.c | 21 ++-------------------
1 file changed, 2 insertions(+), 19 deletions(-)

diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
index 2151bc7f87ab..e1b6e1b51d41 100644
--- a/drivers/vfio/vfio.c
+++ b/drivers/vfio/vfio.c
@@ -546,14 +546,12 @@ struct vfio_device *vfio_group_create_device(struct vfio_group *group,

kref_init(&device->kref);
device->dev = dev;
+ /* Our reference on group is moved to the device */
device->group = group;
device->ops = ops;
device->device_data = device_data;
dev_set_drvdata(dev, device);

- /* No need to get group_lock, caller has group reference */
- vfio_group_get(group);
-
mutex_lock(&group->device_lock);
list_add(&device->group_next, &group->device_list);
group->dev_counter++;
@@ -585,13 +583,11 @@ void vfio_device_put(struct vfio_device *device)
{
struct vfio_group *group = device->group;
kref_put_mutex(&device->kref, vfio_device_release, &group->device_lock);
- vfio_group_put(group);
}
EXPORT_SYMBOL_GPL(vfio_device_put);

static void vfio_device_get(struct vfio_device *device)
{
- vfio_group_get(device->group);
kref_get(&device->kref);
}

@@ -841,14 +837,6 @@ int vfio_add_group_dev(struct device *dev,
vfio_group_put(group);
return PTR_ERR(device);
}
-
- /*
- * Drop all but the vfio_device reference. The vfio_device holds
- * a reference to the vfio_group, which holds a reference to the
- * iommu_group.
- */
- vfio_group_put(group);
-
return 0;
}
EXPORT_SYMBOL_GPL(vfio_add_group_dev);
@@ -928,12 +916,6 @@ void *vfio_del_group_dev(struct device *dev)
unsigned int i = 0;
bool interrupted = false;

- /*
- * The group exists so long as we have a device reference. Get
- * a group reference and use it to scan for the device going away.
- */
- vfio_group_get(group);
-
/*
* When the device is removed from the group, the group suddenly
* becomes non-viable; the device has a driver (until the unbind
@@ -1008,6 +990,7 @@ void *vfio_del_group_dev(struct device *dev)
if (list_empty(&group->device_list))
wait_event(group->container_q, !group->container);

+ /* Matches the get in vfio_group_create_device() */
vfio_group_put(group);

return device_data;
--
2.35.1