[PATCHv2 08/10] vfio/mdev: Improve the create/remove sequence

[Parav Pandit]

This patch addresses below two issues and prepares the code to address
3rd issue listed below.

1. mdev device is placed on the mdev bus before it is created in the
vendor driver. Once a device is placed on the mdev bus without creating
its supporting underlying vendor device, mdev driver's probe() gets triggered.
However there isn't a stable mdev available to work on.

   create_store()
     mdev_create_device()
       device_register()
          ...
         vfio_mdev_probe()
        [...]
        parent->ops->create()
          vfio_ap_mdev_create()
            mdev_set_drvdata(mdev, matrix_mdev);
            /* Valid pointer set above */

Due to this way of initialization, mdev driver who want to use the mdev,
doesn't have a valid mdev to work on.

2. Current creation sequence is,
   parent->ops_create()
   groups_register()

Remove sequence is,
   parent->ops->remove()
   groups_unregister()

However, remove sequence should be exact mirror of creation sequence.
Once this is achieved, all users of the mdev will be terminated first
before removing underlying vendor device.
(Follow standard linux driver model).
At that point vendor's remove() ops shouldn't failed because device is
taken off the bus that should terminate the users.

3. When remove operation fails, mdev sysfs removal attempts to add the
file back on already removed device. Following call trace [1] is observed.

[1] call trace:
kernel: WARNING: CPU: 2 PID: 9348 at fs/sysfs/file.c:327 sysfs_create_file_ns+0x7f/0x90
kernel: CPU: 2 PID: 9348 Comm: bash Kdump: loaded Not tainted 5.1.0-rc6-vdevbus+ #6
kernel: Hardware name: Supermicro SYS-6028U-TR4+/X10DRU-i+, BIOS 2.0b 08/09/2016
kernel: RIP: 0010:sysfs_create_file_ns+0x7f/0x90
kernel: Call Trace:
kernel: remove_store+0xdc/0x100 [mdev]
kernel: kernfs_fop_write+0x113/0x1a0
kernel: vfs_write+0xad/0x1b0
kernel: ksys_write+0x5a/0xe0
kernel: do_syscall_64+0x5a/0x210
kernel: entry_SYSCALL_64_after_hwframe+0x49/0xbe

Therefore, mdev core is improved in following ways.

1. Before placing mdev devices on the bus, perform vendor drivers
creation which supports the mdev creation.
This ensures that mdev specific all necessary fields are initialized
before a given mdev can be accessed by bus driver.
This follows standard Linux kernel bus and device model similar to other
widely used PCI bus.

2. During remove flow, first remove the device from the bus. This
ensures that any bus specific devices and data is cleared.
Once device is taken of the mdev bus, perform remove() of mdev from the
vendor driver.

3. Linux core device model provides way to register and auto unregister
the device sysfs attribute groups at dev->groups.
Make use of this groups to let core create the groups and simplify code
to avoid explicit groups creation and removal.

A below stack dump of a mdev device remove process also ensures that
vfio driver guards against device removal already in use.

 cat /proc/21962/stack
[<0>] vfio_del_group_dev+0x216/0x3c0 [vfio]
[<0>] mdev_remove+0x21/0x40 [mdev]
[<0>] device_release_driver_internal+0xe8/0x1b0
[<0>] bus_remove_device+0xf9/0x170
[<0>] device_del+0x168/0x350
[<0>] mdev_device_remove_common+0x1d/0x50 [mdev]
[<0>] mdev_device_remove+0x8c/0xd0 [mdev]
[<0>] remove_store+0x71/0x90 [mdev]
[<0>] kernfs_fop_write+0x113/0x1a0
[<0>] vfs_write+0xad/0x1b0
[<0>] ksys_write+0x5a/0xe0
[<0>] do_syscall_64+0x5a/0x210
[<0>] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[<0>] 0xffffffffffffffff

This prepares the code to eliminate calling device_create_file() in
subsquent patch.

Signed-off-by: Parav Pandit <parav@xxxxxxxxxxxx>
---
 drivers/vfio/mdev/mdev_core.c    | 94 +++++++++-----------------------
 drivers/vfio/mdev/mdev_private.h |  2 +-
 drivers/vfio/mdev/mdev_sysfs.c   |  2 +-
 3 files changed, 27 insertions(+), 71 deletions(-)

diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
index 1040a4a2dcbc..2b98da2ee361 100644
--- a/drivers/vfio/mdev/mdev_core.c
+++ b/drivers/vfio/mdev/mdev_core.c
@@ -102,55 +102,10 @@ static void mdev_put_parent(struct mdev_parent *parent)
 		kref_put(&parent->ref, mdev_release_parent);
 }
 
-static int mdev_device_create_ops(struct kobject *kobj,
-				  struct mdev_device *mdev)
-{
-	struct mdev_parent *parent = mdev->parent;
-	int ret;
-
-	ret = parent->ops->create(kobj, mdev);
-	if (ret)
-		return ret;
-
-	ret = sysfs_create_groups(&mdev->dev.kobj,
-				  parent->ops->mdev_attr_groups);
-	if (ret)
-		parent->ops->remove(mdev);
-
-	return ret;
-}
-
-/*
- * mdev_device_remove_ops gets called from sysfs's 'remove' and when parent
- * device is being unregistered from mdev device framework.
- * - 'force_remove' is set to 'false' when called from sysfs's 'remove' which
- *   indicates that if the mdev device is active, used by VMM or userspace
- *   application, vendor driver could return error then don't remove the device.
- * - 'force_remove' is set to 'true' when called from mdev_unregister_device()
- *   which indicate that parent device is being removed from mdev device
- *   framework so remove mdev device forcefully.
- */
-static int mdev_device_remove_ops(struct mdev_device *mdev, bool force_remove)
-{
-	struct mdev_parent *parent = mdev->parent;
-	int ret;
-
-	/*
-	 * Vendor driver can return error if VMM or userspace application is
-	 * using this mdev device.
-	 */
-	ret = parent->ops->remove(mdev);
-	if (ret && !force_remove)
-		return ret;
-
-	sysfs_remove_groups(&mdev->dev.kobj, parent->ops->mdev_attr_groups);
-	return 0;
-}
-
 static int mdev_device_remove_cb(struct device *dev, void *data)
 {
 	if (dev_is_mdev(dev))
-		mdev_device_remove(dev, true);
+		mdev_device_remove(dev);
 
 	return 0;
 }
@@ -310,41 +265,43 @@ int mdev_device_create(struct kobject *kobj,
 
 	mdev->parent = parent;
 
+	device_initialize(&mdev->dev);
 	mdev->dev.parent  = dev;
 	mdev->dev.bus     = &mdev_bus_type;
 	mdev->dev.release = mdev_device_release;
 	dev_set_name(&mdev->dev, "%pUl", uuid);
+	mdev->dev.groups = parent->ops->mdev_attr_groups;
+	mdev->type_kobj = kobj;
 
-	ret = device_register(&mdev->dev);
-	if (ret) {
-		put_device(&mdev->dev);
-		goto mdev_fail;
-	}
+	ret = parent->ops->create(kobj, mdev);
+	if (ret)
+		goto ops_create_fail;
 
-	ret = mdev_device_create_ops(kobj, mdev);
+	ret = device_add(&mdev->dev);
 	if (ret)
-		goto create_fail;
+		goto add_fail;
 
 	ret = mdev_create_sysfs_files(&mdev->dev, type);
-	if (ret) {
-		mdev_device_remove_ops(mdev, true);
-		goto create_fail;
-	}
+	if (ret)
+		goto sysfs_fail;
 
-	mdev->type_kobj = kobj;
 	mdev->active = true;
 	dev_dbg(&mdev->dev, "MDEV: created\n");
 
 	return 0;
 
-create_fail:
-	device_unregister(&mdev->dev);
+sysfs_fail:
+	device_del(&mdev->dev);
+add_fail:
+	parent->ops->remove(mdev);
+ops_create_fail:
+	put_device(&mdev->dev);
 mdev_fail:
 	mdev_put_parent(parent);
 	return ret;
 }
 
-int mdev_device_remove(struct device *dev, bool force_remove)
+int mdev_device_remove(struct device *dev)
 {
 	struct mdev_device *mdev, *tmp;
 	struct mdev_parent *parent;
@@ -373,16 +330,15 @@ int mdev_device_remove(struct device *dev, bool force_remove)
 	mutex_unlock(&mdev_list_lock);
 
 	type = to_mdev_type(mdev->type_kobj);
+	mdev_remove_sysfs_files(dev, type);
+	device_del(&mdev->dev);
 	parent = mdev->parent;
+	ret = parent->ops->remove(mdev);
+	if (ret)
+		dev_err(&mdev->dev, "Remove failed: err=%d\n", ret);
 
-	ret = mdev_device_remove_ops(mdev, force_remove);
-	if (ret) {
-		mdev->active = true;
-		return ret;
-	}
-
-	mdev_remove_sysfs_files(dev, type);
-	device_unregister(dev);
+	/* Balances with device_initialize() */
+	put_device(&mdev->dev);
 	mdev_put_parent(parent);
 
 	return 0;
diff --git a/drivers/vfio/mdev/mdev_private.h b/drivers/vfio/mdev/mdev_private.h
index ddcf9c72bd8a..067dc5d8c5de 100644
--- a/drivers/vfio/mdev/mdev_private.h
+++ b/drivers/vfio/mdev/mdev_private.h
@@ -59,6 +59,6 @@ void mdev_remove_sysfs_files(struct device *dev, struct mdev_type *type);
 
 int  mdev_device_create(struct kobject *kobj,
 			struct device *dev, const guid_t *uuid);
-int  mdev_device_remove(struct device *dev, bool force_remove);
+int  mdev_device_remove(struct device *dev);
 
 #endif /* MDEV_PRIVATE_H */
diff --git a/drivers/vfio/mdev/mdev_sysfs.c b/drivers/vfio/mdev/mdev_sysfs.c
index cbf94b8165ea..9f774b91d275 100644
--- a/drivers/vfio/mdev/mdev_sysfs.c
+++ b/drivers/vfio/mdev/mdev_sysfs.c
@@ -236,7 +236,7 @@ static ssize_t remove_store(struct device *dev, struct device_attribute *attr,
 	if (val && device_remove_file_self(dev, attr)) {
 		int ret;
 
-		ret = mdev_device_remove(dev, false);
+		ret = mdev_device_remove(dev);
 		if (ret) {
 			device_create_file(dev, attr);
 			return ret;
-- 
2.19.2