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SubjectRe: [PATCHv1 6/7] vfio/mdev: Fix aborting mdev child device removal if one fails
On Tue, 26 Mar 2019 22:45:44 -0500
Parav Pandit <parav@mellanox.com> wrote:

> device_for_each_child() stops executing callback function for remaining
> child devices, if callback hits an error.
> Each child mdev device is independent of each other.
> While unregistering parent device, mdev core must remove all child mdev
> devices.
> Therefore, mdev_device_remove_cb() always returns success so that

s/always returns/must always return/ ?

> device_for_each_child doesn't abort if one child removal hits error.
>
> While at it, improve remove and unregister functions for below simplicity.
>
> There isn't need to pass forced flag pointer during mdev parent
> removal which invokes mdev_device_remove(). So simplify the flow.
>
> mdev_device_remove() is called from two paths.
> 1. mdev_unregister_driver()
> mdev_device_remove_cb()
> mdev_device_remove()
> 2. remove_store()
> mdev_device_remove()
>
> When device is removed by user using remote_store(), device under
> removal is mdev device.
> When device is removed during parent device removal using generic child
> iterator, mdev check is already done using dev_is_mdev().

Isn't there still a possible race condition (which you seem to address
with the following patch)? IOW, you cannot remove that loop-under-mutex
yet?

>
> Hence, remove the unnecessary loop in mdev_device_remove().
>
> Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> Signed-off-by: Parav Pandit <parav@mellanox.com>
> ---
> drivers/vfio/mdev/mdev_core.c | 23 +++++------------------
> 1 file changed, 5 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> index 836d319..aefcf34 100644
> --- a/drivers/vfio/mdev/mdev_core.c
> +++ b/drivers/vfio/mdev/mdev_core.c
> @@ -149,10 +149,10 @@ static int mdev_device_remove_ops(struct mdev_device *mdev, bool force_remove)
>

Maybe add

/* only called during parent device unregistration */

to avoid headscratching in the future?

> static int mdev_device_remove_cb(struct device *dev, void *data)
> {
> - if (!dev_is_mdev(dev))
> - return 0;
> + if (dev_is_mdev(dev))
> + mdev_device_remove(dev, true);
>
> - return mdev_device_remove(dev, data ? *(bool *)data : true);
> + return 0;
> }
>
> /*
> @@ -240,7 +240,6 @@ int mdev_register_device(struct device *dev, const struct mdev_parent_ops *ops)
> void mdev_unregister_device(struct device *dev)
> {
> struct mdev_parent *parent;
> - bool force_remove = true;
>
> mutex_lock(&parent_list_lock);
> parent = __find_parent_device(dev);
> @@ -254,8 +253,7 @@ void mdev_unregister_device(struct device *dev)
> list_del(&parent->next);
> class_compat_remove_link(mdev_bus_compat_class, dev, NULL);
>
> - device_for_each_child(dev, (void *)&force_remove,
> - mdev_device_remove_cb);
> + device_for_each_child(dev, NULL, mdev_device_remove_cb);
>
> parent_remove_sysfs_files(parent);
>

Up to this chunk, the patch looks good to me.

> @@ -348,24 +346,13 @@ int mdev_device_create(struct kobject *kobj,
>
> int mdev_device_remove(struct device *dev, bool force_remove)
> {
> - struct mdev_device *mdev, *tmp;
> + struct mdev_device *mdev;
> struct mdev_parent *parent;
> struct mdev_type *type;
> int ret;
>
> mdev = to_mdev_device(dev);
> -
> mutex_lock(&mdev_list_lock);
> - list_for_each_entry(tmp, &mdev_list, next) {
> - if (tmp == mdev)
> - break;
> - }
> -
> - if (tmp != mdev) {
> - mutex_unlock(&mdev_list_lock);
> - return -ENODEV;
> - }
> -
> if (!mdev->active) {
> mutex_unlock(&mdev_list_lock);
> return -EAGAIN;

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