linux-kernel - Re: [PATCHv1 6/7] vfio/mdev: Fix aborting mdev child device removal if one fails

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Date:   Mon, 1 Apr 2019 19:39:16 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Parav Pandit <parav@...lanox.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        kwankhede@...dia.com, alex.williamson@...hat.com, cjia@...dia.com
Subject: Re: [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@...lanox.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@...hat.com>
> Signed-off-by: Parav Pandit <parav@...lanox.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;