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SubjectRe: [PATCH] vfio/mdev: Check globally for duplicate devices
On Tue, 15 May 2018 14:17:04 -0600
Alex Williamson <alex.williamson@redhat.com> wrote:

> When we create an mdev device, we check for duplicates against the
> parent device and return -EEXIST if found, but the mdev device
> namespace is global since we'll link all devices from the bus. We do
> catch this later in sysfs_do_create_link_sd() to return -EEXIST, but
> with it comes a kernel warning and stack trace for trying to create
> duplicate sysfs links, which makes it an undesirable response.
>
> Therefore we should really be looking for duplicates across all mdev
> parent devices, or as implemented here, against our mdev device list.
>
> Notably, mdev_parent.lock really only seems to be serializing device
> creation and removal per parent. I'm not sure if this is necessary,
> mdev vendor drivers could easily provide this serialization if it
> is required, but a side-effect of holding the mdev_list_lock to
> protect the namespace is actually greater serialization across the
> create and remove paths, so mdev_parent.lock is removed. If we can
> show that vendor drivers handle the create/remove paths themselves,
> perhaps we can refine the locking granularity.

I'm not sure whether more locking granularity on the create/remove
paths is really worth the effort.

>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
> drivers/vfio/mdev/mdev_core.c | 79 ++++++++++----------------------------
> drivers/vfio/mdev/mdev_private.h | 1
> 2 files changed, 20 insertions(+), 60 deletions(-)

In general, I think this patch makes sense; some nits below.

>
> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> index 126991046eb7..3d8898a2baaf 100644
> --- a/drivers/vfio/mdev/mdev_core.c
> +++ b/drivers/vfio/mdev/mdev_core.c

> @@ -376,12 +346,13 @@ int mdev_device_remove(struct device *dev, bool force_remove)
> struct mdev_device *mdev, *tmp;
> struct mdev_parent *parent;
> struct mdev_type *type;
> - int ret;
> + int ret = 0;

I don't think you need to init this, as ret should either be set to
-ENODEV or the return code of mdev_device_remove_ops(), shouldn't it?

> bool found = false;
>
> mdev = to_mdev_device(dev);
>
> mutex_lock(&mdev_list_lock);
> +

unrelated whitespace change

> list_for_each_entry(tmp, &mdev_list, next) {
> if (tmp == mdev) {
> found = true;
> @@ -389,35 +360,25 @@ int mdev_device_remove(struct device *dev, bool force_remove)
> }
> }
>
> - if (found)
> - list_del(&mdev->next);
> -
> - mutex_unlock(&mdev_list_lock);
> -
> - if (!found)
> - return -ENODEV;
> + if (!found) {
> + ret = -ENODEV;
> + goto out;
> + }
>
> type = to_mdev_type(mdev->type_kobj);
> parent = mdev->parent;
> - mutex_lock(&parent->lock);
>
> ret = mdev_device_remove_ops(mdev, force_remove);
> - if (ret) {
> - mutex_unlock(&parent->lock);
> -
> - mutex_lock(&mdev_list_lock);
> - list_add(&mdev->next, &mdev_list);
> - mutex_unlock(&mdev_list_lock);
> -
> - return ret;
> - }
> + if (ret)
> + goto out;

This change really simplyfies the code, nice.

>
> + list_del(&mdev->next);
> mdev_remove_sysfs_files(dev, type);
> device_unregister(dev);
> - mutex_unlock(&parent->lock);
> mdev_put_parent(parent);
> -
> - return 0;
> +out:
> + mutex_unlock(&mdev_list_lock);
> + return ret;
> }
>
> static int __init mdev_init(void)

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